[AZ-304] C6 Postgres schema: additive 0002 migration + UUIDv5

Strictly additive Alembic migration on the AZ-263 baseline (data_model
.md § 6.1 / § 6.3): six new tiles columns (tile_uuid UNIQUE,
location_hash, content_sha256, disk_bytes, accessed_at, uploaded_at),
four new btree indices, one UNIQUE expression index over the
COALESCE-zero-uuid natural key, CHECK widening of
ck_tiles_freshness_status to the AZ-263 + AZ-303 vocabulary UNION,
four NULLable bbox columns on sector_classifications, and a new
tile_freshness_rules table seeded with the two default thresholds.

Pinned UUIDv5 namespace (TILE_NAMESPACE_UUID =
5b8d0c2e-1a4f-4b3a-8c9d-e7f6a3b2c1d0) + derive_tile_id /
derive_location_hash helpers cross-coordinated with
satellite-provider. Migration runner apply_migrations(config) drives
Alembic command.upgrade("head") against the AZ-263 env with one
retry on PG SQLSTATE 40001 and structured INFO logs on apply / no-op.

Contract bump tile_metadata_store.md v1.1.0 -> v1.2.0 adds
TileMetadata.location_hash: UUID | None = None (non-breaking).
module-layout.md updated so c6_tile_cache explicitly Owns
db/migrations/**.

Tier-1 tests: UUIDv5 determinism + locked vectors + DSN resolution +
retry mocked DBAPIError -> 1180 passed, 32 skipped. Tier-2 docker
schema tests gated by @pytest.mark.docker run against the existing
docker-compose.test.yml db service.

Co-authored-by: Cursor <cursoragent@cursor.com>

db/migrations/env.py

@@ -1,7 +1,11 @@
 """Alembic env.
 
 Bootstrap (AZ-263) ships the minimal `env.py` so `alembic check` resolves
-`0001_initial.sql` as head. Concrete metadata wiring is added by AZ-304.
+`0001_initial` as head. AZ-304 keeps this env minimal: no
+``target_metadata`` is wired because every c6_tile_cache migration uses
+the imperative ``alembic.op.*`` surface (never ``autogenerate``). A
+``target_metadata`` wiring lands the first cycle that adds SQLAlchemy
+ORM models, not before.
 """
 
 from __future__ import annotations