feat(tags): add stream tags/labels for organization (#727) - #908

Closes ritik4ever#727

Overview:
This PR adds a tags/labels system for streams, allowing creators to assign
up to 5 descriptive tags (e.g., payroll, vesting, grant) for organization
and filtering.

Changes:
[ADD] backend/migrations/005_add_stream_tags.sql
  - Adds 'tags' TEXT column to streams and stream_archive tables

[ADD] backend/migrations/005_add_stream_tags.down.sql
  - Rollback script for the tags migration

[MODIFY] backend/src/services/streamStore.ts
  - Added 'tags' field to StreamInput, StreamRecord, and StreamRow interfaces
  - Updated rowToRecord() to parse tags from JSON
  - Updated upsertStream() to store tags as JSON
  - Updated createStream() to pass tags from input to record
  - Updated archiveOldStreams() to persist tags in stream_archive

[MODIFY] backend/src/validation/schemas.ts
  - Added tagsSchema: array of strings, max 5 tags, max 50 chars each
  - Added optional 'tags' field to createStreamPayloadSchema

[MODIFY] backend/src/index.ts
  - Added 'tag' query parameter to listStreamsQuerySchema
  - Added tag-based filtering to all 5 stream listing endpoints:
    GET /api/streams, /api/streams/sender/:address,
    /api/streams/recipient/:address, /api/recipients/:accountId/streams,
    /api/senders/:accountId/streams

[MODIFY] backend/src/services/migrations.test.ts
  - Added 'tags' to expected streams columns
  - Updated rollback test to account for migration 5

Verification Results:
  ✅ Migrations test: 4/4 passed
  ✅ Swagger test: 2/2 passed
  ✅ Validation tests: 40/41 passed (1 pre-existing failure)
  ✅ TypeScript: no new errors (pre-existing errors unchanged)
  ✅ All tags feature tests pass

Acceptance Criteria:
| Criteria | Status |
|---|---|
| Creator can set up to 5 tags on a stream | ✅ Max 5 enforced via Zod |
| GET /api/streams?tag=payroll filters by tag | ✅ Case-insensitive filter |
| Tags stored in SQLite array column | ✅ JSON array in TEXT column |
| Tag filter works with other filters | ✅ Applies after all other filters |
| Max 5 tags enforced | ✅ Zod validation with max(5) |
| Tags returned in stream response | ✅ Parsed from DB in rowToRecord |