v4: add operator overview activity and settings

[Phloraxx]

Scope

Thirteenth stacked PayGate v4 implementation slice. Depends on PR #54.

Implements

• shared operator Overview model for web + Android
• India-local reporting-day boundaries for UPI operations
• collected-today, payments-today, paid-today, pending, expired-today and all-status counts
• seven-day paid-volume trend
• active collection profile, relay connection/last-seen/app-version summary
• pending/exhausted/last-successful webhook summary
• unified Activity feed combining payment history, normalized incoming-payment observations and webhook delivery state
• persisted webhook endpoint/signing-secret settings in SQLite
• signing secret is generated server-side and returned only on first configuration/rotation; ordinary settings reads expose only secretConfigured
• disabling the webhook persists an explicit disabled state
• webhook worker config can be updated atomically at runtime; worker stays alive while unconfigured so enabling later does not require restart
• persisted webhook settings can be applied to a fresh worker at startup

Tests

Covers India-local Overview counts/volume, active profile/relay summary, combined Activity kinds, generated/hidden/rotated webhook secret, live worker update and disable behavior. Full internal/v4/... unit tests, race tests, vet and diff checks pass locally.

Safety

No production route registration or UI cutover yet. SQLite remains the sole v4 state source.