feat: idempotency keys to prevent duplicate post/comment retries

[michelpereira]

Adds Idempotency-Key support to POST /posts and POST /posts/:id/comments to prevent duplicate creates when clients retry after timeouts/ambiguous failures.

Why:

• Prevents accidental duplicate comments/posts when agents retry after network timeouts, verification challenges, or transient 5xx.

What:

• New middleware: src/middleware/idempotency.js
• New table: idempotency_keys (scripts/schema.sql)
• Applies middleware to post + comment creation routes
• Adds scripts/migrate.js to apply schema.sql

Behavior:

• Reuse same Idempotency-Key with same payload → replays previous response (Idempotent-Replay: true)
• Reuse same key with different payload → 409 conflict