Add GET /collections browse endpoint; fix embedded collection poster_path

Completes browse/discover across all five entity types: movies, series,
people, companies, and now collections. No filter (no categorical field
like company_type exists for a collection) -- just page/page_size,
always alphabetical by name.

While adding poster_path to CollectionBriefOut for the browse listing,
found the same class of bug fixed earlier for company logo_path:
movie_service's embedded `collection` field never looked up its poster
artwork either, so it stayed null even after ingest_collection populated
one. Fixed alongside since it's the same schema field this feature was
already touching.

Also removed PageMeta from schemas/common.py -- scaffolding from early
spec work that was never actually wired into any endpoint; all five
browse endpoints shipped this session use a flat page/page_size/
total_results/total_pages shape instead.

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

app/schemas/collection.py

@@ -6,6 +6,7 @@ from datetime import datetime
 
 from pydantic import BaseModel
 
+from app.schemas.common import CollectionBriefOut
 from app.schemas.movie import MovieSummaryOut
 from app.schemas.tv import SeriesSummaryOut
 
@@ -22,3 +23,11 @@ class CollectionDetailOut(BaseModel):
 
     created_at: datetime
     updated_at: datetime
+
+
+class CollectionListOut(BaseModel):
+    page: int
+    page_size: int
+    total_results: int
+    total_pages: int
+    results: list[CollectionBriefOut] = []