Adds /api/v2/narrative, serving a one-sentence summary of how unusual a given
day was at a location. Generation is a batch job (scripts/generate_narratives.py)
that talks to a local Ollama; the endpoint only ever reads the cache, so no
request path depends on the inference host being reachable or powered on. A miss
answers 200 with narrative:null and status "pending" rather than 404.
The model is handed tier labels and percentile directions only, never a
temperature or rainfall figure, so it has nothing to misreport; the sanitizer
then rejects any output that carries units, a forecast or advice. Fact lines
state direction ("warmer than 82% of nights at this time of year here") because
the earlier "82nd percentile (High)" form was read backwards and phrased a warm
night as a cool one. Narratives are unit-free as a result, which also keeps one
cached row per cell/date instead of a per-country degree variant.
Storage is a new `narrative` table in the derived store, LOGGED on Postgres
unlike its two UNLOGGED siblings: losing it costs an LLM pass over the whole
grid, not a recompute from parquet.
