Add world content persistence backend (Postgres chunks, internal API, game-server token auth)

Implements docs/architecture/world_persistence_impl.md §9's backend task:
chunk/room/revision Postgres schema (migration 0005), a Python port of the
world.md §3.6 binary chunk codec with golden-file tests, a WorldStore
service (world.md §5.1 minus flush_dirty/rebuild_index), the public
world-content router (rooms/chunks CRUD, optimistic concurrency, batch
writes) and the internal read-only API for dedicated servers, and
game_servers.py changes: a per-spawn bearer token, --api-url/--server-token
args, and a real "ready"-field wait replacing _await_port, which bound the
UDP port before content existed and failed open (no error) past its
deadline.

create_world now auto-creates the "main" room.

Deviations/findings, not silently decided:
- chunk_codec.py implements real IEEE-754 float32 packing rather than
  mirroring a confirmed bug in scripts/chunk_data_decoder.gd's
  _read_float32/_write_float32, which do a lossy int<->float cast instead
  of a bit reinterpretation (verified against Godot 4.6.2 headless) and
  will corrupt any non-integer position/rotation/scale/bounds value. Not
  fixed here (client-side, out of scope); flagged for the client task.
- world_persistence_impl.md references a world's monotonic
  content_revision (§6/§7.2/§8.1) but never defines where it's stored.
  Resolved as MAX(id) over the append-only world_chunk_revisions table,
  scoped to world_id, rather than adding an undocumented column.
- No existing Godot-side golden fixture/test vectors were found to reuse,
  so tests/fixtures/chunks/*.bin are authored fresh from this
  implementation (see generate_fixtures.py).

Verification: 132/132 backend pytest tests pass against a real ephemeral
Postgres (both reusing an already-running instance and cold via
conftest.py's own docker-compose bootstrap), including migration
upgrade/downgrade/upgrade, codec round-trip + validation-error tests, and
full HTTP round trips for PUT/GET/batch/delete/revisions/rollback and the
internal API's token auth.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y2FESCaMXeJpC9gSctiEeD

backend/app/main.py

@@ -6,7 +6,7 @@ from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
 from app.config import settings
-from app.routers import auth, users, worlds, assets, avatar
+from app.routers import auth, users, worlds, assets, avatar, world_content, internal
 
 
 from app.services.game_servers import supervisor
@@ -40,8 +40,10 @@ app.add_middleware(
 app.include_router(auth.router, prefix="/api/auth", tags=["auth"])
 app.include_router(users.router, prefix="/api/users", tags=["users"])
 app.include_router(worlds.router, prefix="/api/worlds", tags=["worlds"])
+app.include_router(world_content.router, prefix="/api/worlds", tags=["world-content"])
 app.include_router(assets.router, prefix="/api/assets", tags=["assets"])
 app.include_router(avatar.router, prefix="/api/avatar", tags=["avatar"])
+app.include_router(internal.router, prefix="/api/internal", tags=["internal"])
 
 
 @app.get("/api/health")