chore(app-api): remove dead sessions/services/metadata.py duplicate

[philmerrell]

Summary

backend/src/apis/app_api/sessions/services/metadata.py was a 1060-line near-verbatim copy of backend/src/apis/shared/sessions/metadata.py, left behind by #200. This PR verifies it is unreachable and deletes it.

Why it matters beyond tidiness: the duplicate still wrote the legacy rotating session sort key (S#ACTIVE#{last_message_at}#{session_id} / S#DELETED#…), which the shared module migrated away from in #175. Any code path that reached it would have written rows invisible to the SessionRecencyIndex (GSI4) listing. docs/specs/session-metadata-static-sort-key.md already called it a dead duplicate.

Verification that it was dead

• Importers: repo-wide grep for app_api.sessions.services.metadata, services.metadata, from .metadata, services/metadata → only the five call sites in test_cache_savings.py. stream_coordinator.py, quota/checker.py, display_text.py, chat/admin/session routes all import apis.shared.sessions.metadata.
• Dynamic imports: every importlib / import_module / __import__ hit in the repo (incl. backend/src/lambdas/, backend/scripts/, workflows, Dockerfiles) is unrelated. No string-path reference anywhere.
• Re-exports: sessions/services/__init__.py is empty; sessions/__init__.py exports only router.
• Symbols: every top-level def in the app_api copy has a same-named counterpart in the shared copy. _update_cost_summary_async has an identical signature; the shared version additionally has the _coerce_cost_total helper and or 0 guards for None cache pricing.

Changes

• Delete backend/src/apis/app_api/sessions/services/metadata.py
• Repoint the five imports in backend/src/apis/app_api/sessions/tests/test_cache_savings.py at apis.shared.sessions.metadata. The existing patch('apis.shared.storage.get_metadata_storage') needed no change since both copies import the storage factory lazily inside the function.
• Fix three spec citations that would have dangled (agent-marketplace.md had a real relative link to the deleted file; gpt-5-6-prompt-caching.md and quota-cooldown-windows.md cited line numbers). Repointed to the shared module.

Test plan

• uv run pytest src/apis/app_api/sessions/tests/ tests/costs/ → 220 passed
• uv run pytest tests/ (the CI command) → 8345 passed, 3 skipped
• python -c "import apis.app_api.sessions.services.metadata" → ModuleNotFoundError

Note for reviewers

test_cache_savings.py lives under src/, and both pytest.ini and pyproject.toml set testpaths = tests, so it is not collected in CI today (nor are the other in-src/ test dirs). It passes locally by explicit path. Moving it to backend/tests/costs/ would get it into CI now that it exercises the shared module; left out of this PR as a scope expansion.

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