fix(tabular): TR-02 to TR-05 bugs + SSE keepalive heartbeat

From the PR description

Fix: Tabular Review bugs + SSE keepalive

Bugs fixed

BUG-TR-01: ALB routing intercepts frontend routes (infra, not in this diff)

• Added header-based ALB listener rules (P5-P8) for /tabular-review and /single-documents
• Browser navigation (Accept: text/html) → Frontend; API calls → Backend

BUG-TR-02: missingModelApiKey ignores env key fallback

• Added hasEnvApiKey() check in missingModelApiKey() as defensive fallback
• Note: getUserApiKeys() already includes env keys, but this guards against future refactors

BUG-TR-03: Generation ignores document_ids from review

• POST /:reviewId/generate now uses review.document_ids as primary source of truth
• Falls back to cell-derived doc IDs only if document_ids is not set

BUG-TR-04: Silent failure on documents without storage_path

• If extracted markdown is empty, cells are now marked as error with descriptive message
• Prevents LLM from receiving empty text and returning "Not Found" as if analyzed

BUG-TR-05: JSON parsing breaks on markdown code fences

• processLine now strips ```json ... ``` fences before parsing
• Falls back to regex extraction of first JSON object if line doesn't start with {

Also included: SSE keepalive heartbeat

• chat.ts and projectChat.ts now send : keepalive SSE comments every 15s
• Prevents ALB idle timeout (60s, now 300s) from dropping connections during long tool executions