polar: build /api/polar/webhook handler

Standalone router for inbound Polar (merchant-of-record) deliveries.
No bearer-token dep — authenticity comes from the Standard Webhooks
HMAC instead. Wired up so it's safe to deploy dark: empty
POLAR_WEBHOOK_SECRET makes the endpoint return 503 (loud) rather than
accept unsigned events.

Behaviour
- Standard Webhooks signature verification: HMAC-SHA256 over
  `{webhook-id}.{webhook-timestamp}.{body}`, base64 secret prefixed
  whsec_, ±5min replay window, constant-time compare against any of
  the space-separated v1 tokens.
- Idempotency via UNIQUE on polar_events.event_id — a replayed
  webhook-id short-circuits to 200 "duplicate" without re-running.
- Event dispatch table covers the 10 events we subscribed to:
  subscription.{created,active,updated,uncanceled} -> tier=paid +
  persist polar_customer_id / polar_subscription_id.
  subscription.revoked -> tier=free (customer id kept so a resub
  matches the same User row).
  canceled / past_due / order.* / refund.created -> audit only.
- Unknown event types are acked 200 + recorded; we don't want to 4xx
  on something Polar adds in the future and trigger their retry loop.

Schema (migration 0018)
- users.polar_customer_id, users.polar_subscription_id (both nullable
  String(64)); UNIQUE on polar_customer_id so two users can't claim
  the same Polar identity.
- polar_events table: event_id (unique), event_type, received_at,
  processed_at, error, raw payload (truncated to 16 KiB).

Tests
- 7 in tests/test_polar_webhook.py: bad signature -> 401, stale
  timestamp -> 401, missing headers -> 400, subscription.active flips
  tier to paid + stores IDs, subscription.revoked drops to free while
  keeping customer link, replayed webhook-id is no-op, unknown event
  is acked.
- Full suite: 212 passed, 5 skipped.

Operator next steps before saving the webhook in Polar
1. Pull this branch to prod and apply migration 0018.
2. Save the webhook in Polar pointing at
   https://read.markets/api/polar/webhook — Polar will accept the
   save even though our endpoint still 503s (no secret yet).
3. Copy the secret Polar reveals into the prod .env as
   POLAR_WEBHOOK_SECRET=whsec_... and restart the app.
4. Trigger a test event from Polar's dashboard to confirm 200 OK.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/config.py

@@ -92,6 +92,13 @@ class Settings(BaseSettings):
     CASSANDRA_ANALYSIS: str = "SPECULATIVE"   # DRY | SPECULATIVE
     BETA_MODE: bool = True  # Shows a "BETA" pill in the app header. Flip to False at GA.
 
+    # Polar (merchant-of-record). Webhook secret is base64-encoded with a
+    # `whsec_` prefix in the Polar dashboard; paste it verbatim into the
+    # env var. Empty = webhook endpoint refuses with 503 (so a misconfig
+    # is loud rather than silently accepting unsigned events).
+    POLAR_WEBHOOK_SECRET: str = ""
+    POLAR_API_KEY: str = ""
+
     # Config file locations (overridable for tests)
     BASELINE_TOML: Path = Field(default_factory=lambda: CONFIG_DIR / "default.toml")
     PORTFOLIO_TOML: Path = Field(default_factory=lambda: CONFIG_DIR / "portfolio.toml")