feat(feeds): add email feed system with AI-powered content curation

Users can create feed email addresses (feed-<hash>@thestack.cl), subscribe them
to external newsletters, and an AI agent (Gemini 2.5 Flash) processes incoming
emails to publish relevant tech content. Posts are published under the user's
name with a "via feed" badge.

- DB: feeds, feed_logs tables + source/feedId/status columns on posts
- API: CRUD feeds, pending posts approval/rejection, admin feed management
- Email: Cloudflare Email Routing handler with postal-mime parsing
- Agent: LangChain agent with fetch_url/publish_post/skip tools
- Frontend: feeds management in settings, pending posts page, admin feeds tab
- Rate limits: 3 feeds per user, 1 email per 3 days per feed

Author: emersoftware

api/migrations/0012_create_feeds.sql

@@ -0,0 +1,34 @@
+CREATE TABLE feeds (
+  id TEXT PRIMARY KEY,
+  user_id TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  name TEXT NOT NULL,
+  email_hash TEXT NOT NULL UNIQUE,
+  auto_publish INTEGER NOT NULL DEFAULT 0,
+  is_active INTEGER NOT NULL DEFAULT 1,
+  last_processed_at INTEGER,
+  created_at INTEGER NOT NULL,
+  updated_at INTEGER NOT NULL
+);
+
+CREATE INDEX idx_feeds_user ON feeds(user_id);
+CREATE UNIQUE INDEX idx_feeds_email_hash ON feeds(email_hash);
+
+CREATE TABLE feed_logs (
+  id TEXT PRIMARY KEY,
+  feed_id TEXT NOT NULL REFERENCES feeds(id) ON DELETE CASCADE,
+  email_subject TEXT,
+  email_from TEXT,
+  links_found INTEGER DEFAULT 0,
+  links_published INTEGER DEFAULT 0,
+  links_skipped INTEGER DEFAULT 0,
+  status TEXT NOT NULL DEFAULT 'processing',
+  error TEXT,
+  created_at INTEGER NOT NULL
+);
+
+CREATE INDEX idx_feed_logs_feed ON feed_logs(feed_id);
+CREATE INDEX idx_feed_logs_created_at ON feed_logs(created_at);
+
+ALTER TABLE posts ADD COLUMN source TEXT;
+ALTER TABLE posts ADD COLUMN feed_id TEXT REFERENCES feeds(id) ON DELETE SET NULL;
+ALTER TABLE posts ADD COLUMN status TEXT NOT NULL DEFAULT 'published';