Add GET /genres directory endpoint

GET /movies and GET /series accept a genre filter (an Aurora genre id),
but nothing exposed what those ids actually are -- genres otherwise
only ever appear embedded in a movie/series response's own genres
list. This is the resolve-by-name-first-then-filter step the frontend
needs to build real genre browsing.

No filter, no sort_by -- always alphabetical by name, same pattern as
collections/companies (genres have no categorical field or popularity/
date to sort by). page_size defaults to 50 (max 200) rather than the
20/50 used elsewhere, since genres are a small, fixed reference set
(TMDb has ~19-20 total) a client typically wants in full for a filter
dropdown, not paginated like a catalog of millions of movies.

Also fixed two stale "behind Traefik" mentions in app/main.py's module
docstring while touching this file, left over from the earlier
cPanel/host-webserver deployment change.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

docs/api.md

@@ -178,9 +178,23 @@ date field to sort by, so results are always alphabetical by name.
 
 `GET /api/v1/collections` is the same idea for collections/franchises, but
 with no filter at all (no categorical field like `company_type` exists for
-a collection) — just `page`/`page_size`, always alphabetical by name. This
-is the last entity type to get a browse endpoint: movies, series, people,
-companies, and collections are all browsable now.
+a collection) — just `page`/`page_size`, always alphabetical by name.
+
+## Genres
+
+| Method | Path |
+|---|---|
+| GET | `/api/v1/genres` |
+
+A directory for the `genre` filter `GET /movies` and `GET /series` already
+accept, which takes an Aurora genre id — this is how a client discovers
+what those ids actually are, since nothing else exposes them on their own
+(they otherwise only ever appear embedded in a movie/series response's own
+`genres` list). No filter, no `sort_by` — always alphabetical by name, same
+as collections. `page_size` defaults to 50 (max 200) rather than the 20/50
+used elsewhere, since genres are a small, fixed reference set (TMDb has
+~19-20 total) that a client typically wants in full for a filter dropdown,
+not paginated the way a catalog of millions of movies needs to be.
 
 Collection detail's `movies` list only includes movies already ingested
 into Aurora — `POST /api/v1/admin/ingest/collection` links a franchise's