Extract the glossary and static-page SEO copy into a schema-validated content/ tree (#241)

The weather-terms glossary (9 entries) and four static pages' SEO title/
description were hardcoded directly in web/content.py - a 1019-line module
that also owns all SSR rendering logic - mixed in with code that changes on
a completely different cadence and for different reasons.

Add content/glossary.yaml and content/pages.yaml (a new repo-root content/
tree, a sibling of backend/ and frontend/ paths.py resolves the same way -
the seed of a future thermograph-copy repo per the architecture decision
doc's own §4) plus web/content_loader.py: a small loader that validates each
file's shape at load time (required fields present and non-empty, no
duplicate glossary slugs) and fails loudly on a malformed edit rather than
rendering a blank glossary card or an empty <title>. content.py's GLOSSARY
dict and the about/privacy/hub/glossary_index page_title/description
literals now come from the loader.

Scope: only content that is genuinely pure static data with no embedded
template logic. cities_flavor.json (Wikipedia extracts, already its own
generated file) and the homepage's title/description (embedded in
home.html.j2 as Jinja block overrides, a heavily-tested product-critical
template) are deliberately left as they are - a future pass, not required
for this one. UI microcopy bound to frontend logic stays in frontend/,
per the doc's own line between content and frontend.

Verified: content/glossary.yaml generated programmatically from the live
GLOSSARY dict (not hand-transcribed) and round-tripped byte-for-byte
identical against it; content/pages.yaml's four entries checked field-by-
field against the original hardcoded strings. Full backend suite green (362
passed, 4 skipped) with zero existing test changes needed beyond one
assertion made escaping-aware (a pre-existing Jinja double-escape quirk on
the one title containing "&amp;", intentionally preserved not fixed). Built
and booted the real Docker image: content/ present at /app/content, and
curled /glossary, /glossary/percentine, /about, /privacy from inside the
running container - all four render with the exact expected title text.

content/glossary.yaml

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+# Weather-terms glossary. Each entry: slug, term (display name), short (one-line
+# definition), body (full HTML; {base} is replaced with the app base path at load
+# time). Consumed by backend/web/content_loader.py -- see docs/README.md.
+terms:
+- slug: climate-normal
+  term: Climate normal
+  short: The typical value of a weather metric for a place and time of year, averaged over decades.
+  body: |-
+    A <b>climate normal</b> is the long-term average of a weather variable (say, the daily high) for a specific place and time of year. Thermograph builds each day's normal from every historical day within &plusmn;7 days of that day-of-year, across ~45 years, so a day is judged against its own season, not a single annual average.
+- slug: percentile
+  term: Percentile
+  short: 'Where a value ranks within a distribution: the 90th percentile is warmer than 90% of days.'
+  body: |-
+    A <b>percentile</b> says where a value falls within a range of past values. If today's high is at the 97th percentile, only about 3% of comparable days in this location's history were warmer. Thermograph grades every day by its percentile against the local &plusmn;7-day seasonal distribution.
+- slug: temperature-anomaly
+  term: Temperature anomaly
+  short: 'How far a temperature departs from normal: the difference from the long-term average.'
+  body: |-
+    A <b>temperature anomaly</b> is how much warmer or colder it is than the local normal for the time of year. Thermograph expresses the same idea as a percentile and a grade (from “Below Normal” to “Near Record”), so an anomaly is easy to read at a glance for any location.
+- slug: feels-like
+  term: Feels-like temperature
+  short: What the air actually feels like once humidity and wind are accounted for.
+  body: |-
+    <b>Feels-like</b> (apparent temperature) combines air temperature with humidity and wind. In heat it uses the <a href="{base}/glossary/heat-index">heat index</a>; in cold it uses <a href="{base}/glossary/wind-chill">wind chill</a>. Thermograph grades feels-like against its own local history, so “Near Record” means extreme <i>for that place</i>.
+- slug: heat-index
+  term: Heat index
+  short: How hot it feels when humidity is factored into the air temperature.
+  body: |-
+    The <b>heat index</b> is the apparent temperature on a hot, humid day: high humidity slows sweat evaporation, so it feels hotter than the thermometer reads. Thermograph folds it into the <a href="{base}/glossary/feels-like">feels-like</a> metric and grades how unusual it is locally.
+- slug: wind-chill
+  term: Wind chill
+  short: How cold it feels when wind is factored into the air temperature.
+  body: |-
+    <b>Wind chill</b> is the apparent temperature on a cold, windy day: wind strips away body heat, so it feels colder than the air temperature. It's the cold-weather half of <a href="{base}/glossary/feels-like">feels-like</a>.
+- slug: humidity
+  term: Absolute humidity
+  short: The actual mass of water vapor in the air, in grams per cubic meter.
+  body: |-
+    Thermograph reports <b>absolute humidity</b> (g/m&sup3;), the real amount of water vapor in the air, rather than relative humidity, which shifts with temperature. Graded against local history, it shows genuinely muggy or unusually dry days.
+- slug: reanalysis
+  term: Reanalysis (ERA5)
+  short: A gridded, physically-consistent record of past weather for anywhere on Earth.
+  body: |-
+    A <b>reanalysis</b> blends historical observations with a weather model to produce a consistent record of past conditions everywhere, even where no station exists. Thermograph's ~45-year history comes from ECMWF's <b>ERA5</b> reanalysis (via Open-Meteo), which is why it works for any point on Earth.

content/pages.yaml

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+# SEO title/description for the static pages (not the per-city/per-month pages,
+# whose titles are built dynamically from city data in web/content.py). Each key
+# is the page identifier passed to content_loader.load_page(); see
+# backend/web/content_loader.py and docs/README.md.
+pages:
+  about:
+    title: "About Thermograph: how the weather grades are calculated"
+    description: >-
+      How Thermograph works: ~45 years of ERA5 climate history, a ±7-day
+      seasonal window, and empirical percentiles that grade each day relative to
+      its own location.
+  privacy:
+    title: "Privacy | Thermograph"
+    description: >-
+      What Thermograph does and does not collect: no tracking, no ads, no
+      account required, and location that never leaves your browser.
+  hub:
+    title: "City climate pages: averages, records and how today compares"
+    description: >-
+      Browse climate pages for hundreds of cities worldwide: average
+      temperatures by month, all-time records, and how today's weather compares
+      to local history.
+  glossary_index:
+    title: "Weather & climate glossary: heat index, feels-like, percentile, and more"
+    description: >-
+      Plain-language definitions of weather and climate terms: climate normal,
+      percentile, temperature anomaly, feels-like, heat index, wind chill,
+      humidity, and reanalysis.