fix(webhooks): route webhook endpoints through tenant-aware engine methods

[cdbartholomew]

Summary

The HTTP webhook endpoints (create_webhook, list_webhooks, update_webhook, delete_webhook, list_webhook_deliveries) in hindsight-api-slim/hindsight_api/api/http.py resolved fq_table("webhooks") before any tenant authentication, doing raw pool.fetchrow / pool.fetch calls directly. fq_table reads the target schema from async-local context (get_current_schema()), which is set per-request by _authenticate_tenant. Skipping that step caused these endpoints to fall through to the default schema regardless of any per-request schema resolution.

Under any deployment that uses async-local context to set a per-request target schema (multi-tenant routing), this meant webhook rows were silently written to and read from the default schema while every other operation on the same bank — retain, consolidate, mental-models, etc. — correctly resolved to the per-target schema. The fire path then never matched the webhooks (it looks them up in the bank's resolved schema) and no webhook_delivery async operations were ever enqueued. The failure was completely silent: no errors, the webhook lists/creates/updates fine via the API, but no events ever fire.

Fix

Move the SQL into MemoryEngine methods (create_webhook, list_webhooks, update_webhook, delete_webhook, list_webhook_deliveries) that call _authenticate_tenant(request_context) first — matching the pattern already used by retain, consolidate, mental-models, etc. The HTTP handlers now delegate to those engine methods. fq_table("webhooks") resolves through the same schema contextvar as the rest of the bank's data.

The fire path itself is unchanged — _webhook_manager.fire_event_with_conn already takes its own schema= parameter.

Test plan

• Six new schema-isolation tests in tests/test_webhooks.py covering create / list / update / delete / list-deliveries — each provisions a non-default schema via run_migrations and a swapped _tenant_extension, then asserts the relevant SQL targets the resolved schema and not the default.
• All 31 pre-existing tests/test_webhooks.py cases still pass (37/37 total).
• tests/test_audit_log.py (17/17) and tests/test_schema_isolation.py (4/4) clean.
• One pre-existing failure in test_extensions.py::test_reflect_validation reproduces identically on main — unrelated to this change.

Notes

• No public API contract changes (request/response shapes, status codes, paths all identical).
• No new database migrations.
• Other endpoints in http.py that follow a similar shape (_get_backend() then fq_table(...) without an engine method indirection) were spot-checked but not exhaustively audited — worth a follow-up grep job for from hindsight_api.engine.memory_engine import fq_table inside http.py to confirm webhooks were the only case where the schema context was missed before SQL was emitted.