feat: persist Dead Letter Queue to PostgreSQL (closes #221)

[whiteghost0001]

The Dead Letter Queue (DLQ) used by the indexer and webhook subsystems was stored entirely in-memory (a plain array dlqEntries in src/routes/dlq.ts). This meant every dead-lettered event was lost on process restart, making the DLQ useless for production reliability and incident investigation.

What was done

1. Database migration — migrations/1774715300000_dead-letter-queue.ts Created a new node-pg-migrate migration that adds the dead_letter_queue table with the following schema:

• id TEXT PRIMARY KEY
• topic TEXT NOT NULL
• payload JSONB NOT NULL
• error TEXT NOT NULL
• attempts INTEGER NOT NULL DEFAULT 1
• correlation_id TEXT (nullable)
• first_failed_at TIMESTAMPTZ NOT NULL DEFAULT current_timestamp
• last_failed_at TIMESTAMPTZ NOT NULL DEFAULT current_timestamp

Indexes on topic and first_failed_at cover the two most common query patterns (filter by topic, sort by age).

2. DLQ repository — src/db/repositories/dlqRepository.ts New repository following the same pattern as streamRepository.ts: uses getPool() and query() from src/db/pool.ts.

Exposed methods:

• insert(entry) — persists a new DLQ entry
• findAll({ limit, offset, topic? }) — paginated list with optional topic filter; returns { entries, total }
• findById(id) — single entry lookup
• update(id, patch) — partial update for replay (reset attempts / update lastFailedAt)
• deleteById(id) — acknowledge/remove one entry; returns boolean
• deleteAll(topic?) — bulk purge with optional topic filter; returns count of deleted rows

3. Route update — src/routes/dlq.ts

• Removed the in-memory dlqEntries array, getDlqEntries(), and _resetDlq() helpers.
• enqueueDeadLetter() is now async and calls dlqRepository.insert().
• All five route handlers (GET /, GET /:id, POST /:id/replay, DELETE /:id, DELETE /) now delegate to the repository instead of mutating the in-memory array.
• Audit events and structured logging are preserved unchanged.

How it was done

• Followed the existing node-pg-migrate migration format (MigrationBuilder API, timestamp-prefixed filename).
• Followed the existing repository pattern (getPool/query, row-to-type mapper, parameterised queries throughout — no string interpolation of user input).
• TypeScript compiles cleanly (tsc --noEmit); the only pre-existing error is an unrelated syntax issue in tests/webhooks/retry.rateLimit.test.ts.

Closes #221

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