feat(10-02): Google Calendar OAuth endpoints and per-tenant calendar tool

- Add calendar_auth.py: OAuth install/callback/status endpoints with HMAC-signed state
- Replace calendar_lookup.py service account stub with per-tenant OAuth token lookup
- Support list, check_availability, and create actions with natural language responses
- Token auto-refresh: write updated credentials back to channel_connections on refresh
- Add migration 013: add google_calendar to channel_type CHECK constraint
- Add unit tests: 16 tests covering all actions, not-connected path, token refresh write-back

migrations/versions/013_google_calendar_channel.py

@@ -0,0 +1,52 @@
+"""Add google_calendar to channel_type CHECK constraint
+
+Revision ID: 013
+Revises: 012
+Create Date: 2026-03-26
+
+Adds 'google_calendar' to the valid channel types in channel_connections.
+This enables per-tenant Google Calendar OAuth token storage alongside
+existing Slack/WhatsApp/web connections.
+
+Steps:
+  1. Drop old CHECK constraint on channel_connections.channel_type
+  2. Re-create it with the updated list including 'google_calendar'
+"""
+
+from __future__ import annotations
+
+from alembic import op
+
+# Alembic revision identifiers
+revision: str = "013"
+down_revision: str | None = "012"
+branch_labels = None
+depends_on = None
+
+# All valid channel types including 'google_calendar'
+_CHANNEL_TYPES = (
+    "slack", "whatsapp", "mattermost", "rocketchat", "teams", "telegram", "signal", "web", "google_calendar"
+)
+
+
+def upgrade() -> None:
+    # Drop the existing CHECK constraint (added in 008_web_chat.py as chk_channel_type)
+    op.execute("ALTER TABLE channel_connections DROP CONSTRAINT IF EXISTS chk_channel_type")
+
+    # Re-create with the updated list
+    op.execute(
+        "ALTER TABLE channel_connections ADD CONSTRAINT chk_channel_type "
+        f"CHECK (channel_type IN {tuple(_CHANNEL_TYPES)})"
+    )
+
+
+def downgrade() -> None:
+    # Restore 008's constraint (without google_calendar)
+    _PREV_TYPES = (
+        "slack", "whatsapp", "mattermost", "rocketchat", "teams", "telegram", "signal", "web"
+    )
+    op.execute("ALTER TABLE channel_connections DROP CONSTRAINT IF EXISTS chk_channel_type")
+    op.execute(
+        "ALTER TABLE channel_connections ADD CONSTRAINT chk_channel_type "
+        f"CHECK (channel_type IN {tuple(_PREV_TYPES)})"
+    )