feat(observability): add merchant thresholds APIs

[VenuMadhav2541]

Summary

Adds PostgreSQL persistence and HTTP APIs for per-merchant threshold overrides in the observability service.

The implementation follows the alert_manager module structure used by the alerts-dicts APIs. IDs are generated by the application and stored as VARCHAR; no UUID or database-generated ID is used.

APIs

• POST /alerts/alerts_manager/merchant_thresholds
  • Inserts a threshold override.
  • Upserts on (name, product, merchant_id, profile_id, is_enabled, author).
  • On conflict, updates only threshold fields, metadata, and timestamp that were supplied.
• POST /alerts/alerts_manager/merchant_thresholds/list
  • Lists rows using optional filters for IDs, alert name, product, merchant, profile, author, enabled state, metadata values, and last_updated_at range.
• POST /alerts/alerts_manager/merchant_thresholds/update
  • Updates all rows matching at least one supplied key filter.
  • Supports absent fields as keep, null as clear, and values as replace.
  • Supports metadata object merge.
• POST /alerts/alerts_manager/merchant_thresholds/delete
  • Deletes rows matching the required name/product and optional merchant/profile filters.
• DELETE /alerts/alerts_manager/merchant_thresholds/{id}
  • Deletes one row by application-generated string ID.

Database

• Adds the merchant_thresholds migration and rollback.
• Uses VARCHAR(64) for the application-generated primary key.
• Adds the Diesel schema, models, queries, and conflict constraint.

Validation and Tests

• Rejects unknown request fields and invalid empty update filters.
• Validates text lengths and blank IDs before database access.
• Verifies route authentication and bad-request responses.
• cargo test -p observability passes: 96 unit tests, 6 merchant-threshold integration tests, 16 notifier tests, and doc tests.

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
crates/observability/src/db.rs	78% smaller
crates/diesel_models/src/observability/schema.rs	50% smaller
crates/observability/src/routes/app.rs	14% smaller
crates/api_models/src/observability.rs	0% smaller
crates/api_models/src/observability/alert_manager.rs	0% smaller
crates/api_models/src/observability/alert_manager/merchant_thresholds.rs	0% smaller
crates/diesel_models/src/observability.rs	0% smaller
crates/diesel_models/src/observability/alert_manager.rs	0% smaller
crates/diesel_models/src/observability/alert_manager/merchant_thresholds.rs	0% smaller
crates/diesel_models/src/observability/query.rs	0% smaller
crates/diesel_models/src/observability/query/alert_manager.rs	0% smaller
crates/diesel_models/src/observability/query/alert_manager/merchant_thresholds.rs	0% smaller
crates/observability/migrations/2026-09-16-000001_create_merchant_thresholds/up.sql	Unsupported file format
crates/observability/src/core.rs	0% smaller
crates/observability/src/core/alert_manager.rs	0% smaller
crates/observability/src/core/alert_manager/merchant_thresholds.rs	0% smaller
crates/observability/src/db/alert_manager.rs	0% smaller
crates/observability/src/db/alert_manager/merchant_thresholds.rs	0% smaller
crates/observability/src/domain_models.rs	0% smaller
crates/observability/src/domain_models/alert_manager.rs	0% smaller
crates/observability/src/domain_models/alert_manager/merchant_thresholds.rs	0% smaller
crates/observability/src/routes.rs	0% smaller
crates/observability/src/routes/alert_manager.rs	0% smaller
crates/observability/src/routes/alert_manager/merchant_thresholds.rs	0% smaller
crates/observability/tests/merchant_thresholds.rs	0% smaller

[XyneSpaces]

[blocking] crates/observability/migrations/2026-09-16-000001_create_merchant_thresholds/up.sql changes the already-defined id column from UUID DEFAULT gen_random_uuid() PRIMARY KEY to VARCHAR(64) NOT NULL PRIMARY KEY.

Editing a migration after it has been introduced means environments that already ran it will have a different schema than new ones. Please revert this change and add a new migration to alter the column type if needed.