Add scheduled social posts, Discord channels config, and LinkedIn personal posting fix

- Scheduled posts: new Schedule tab with multi-platform post scheduling (Discord,
  Mastodon, LinkedIn); SQLite queue with per-target delivery tracking and retry;
  cron script in bin/process-scheduled-posts.php processes due targets inline
- Discord channels: new config section for webhook-based channels used as scheduled
  post targets; webhook URLs stored in .env as DISCORD_WEBHOOK_<KEY>
- LinkedIn: fix botcompose_post to use member_id (urn:li:person) for personal
  profiles instead of hardcoded org author; filter org pages out of compose UI
  and RSS modal picker since org posting is unsupported; label org pages clearly

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sql/009_scheduled_posts.sql

@@ -0,0 +1,26 @@
+-- Scheduled social posts campaign (one row = one scheduled send)
+CREATE TABLE IF NOT EXISTS scheduled_posts (
+    id           INTEGER PRIMARY KEY AUTOINCREMENT,
+    project_id   INTEGER NOT NULL REFERENCES projects(id) ON DELETE CASCADE,
+    content      TEXT NOT NULL,
+    url          TEXT,
+    scheduled_at DATETIME NOT NULL,
+    status       TEXT NOT NULL DEFAULT 'pending',  -- pending | processing | done | partial
+    created_at   DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+-- Per-platform delivery target (one row per platform × target per post)
+CREATE TABLE IF NOT EXISTS scheduled_post_targets (
+    id       INTEGER PRIMARY KEY AUTOINCREMENT,
+    post_id  INTEGER NOT NULL REFERENCES scheduled_posts(id) ON DELETE CASCADE,
+    platform TEXT NOT NULL,  -- discord | mastodon | linkedin
+    target   TEXT NOT NULL,  -- channel key (discord) | account key (mastodon) | page key (linkedin)
+    status   TEXT NOT NULL DEFAULT 'pending',  -- pending | sent | failed
+    error    TEXT,
+    sent_at  DATETIME
+);
+
+CREATE INDEX IF NOT EXISTS idx_scheduled_posts_project_status
+    ON scheduled_posts(project_id, status, scheduled_at);
+CREATE INDEX IF NOT EXISTS idx_scheduled_post_targets_post
+    ON scheduled_post_targets(post_id, status);