Add Social Scheduler — global social media post scheduling

- New /social page with left/right layout (form + post list)
- Social topbar link
- Tables: social_posts, social_targets (project-agnostic posts, per-target project refs)
- CRUD: create, edit, duplicate, delete, retry
- Image upload (JPEG/PNG/GIF/WebP, 10MB max) stored in data/social-uploads/
- Per-platform image toggle (Discord multipart, Mastodon /api/v2/media, LinkedIn Assets API)
- bin/dispatch-social.php cron dispatcher (every minute via /etc/cron.d)
- Settings: eligible projects section to enable bot projects as social account sources
- Platforms: Discord (webhook), Mastodon, LinkedIn personal profile

Cron setup:
  * * * * * www-data /usr/bin/php /opt/hackmancms/bin/dispatch-social.php >> /var/log/hackman_social.log 2>&1

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

sql/010_social_posts.sql

@@ -0,0 +1,24 @@
+-- Global social post scheduler (project-agnostic posts, per-target project refs)
+CREATE TABLE IF NOT EXISTS social_posts (
+    id           INTEGER PRIMARY KEY AUTOINCREMENT,
+    content      TEXT    NOT NULL,
+    image_path   TEXT,
+    scheduled_at TEXT    NOT NULL,
+    status       TEXT    NOT NULL DEFAULT 'pending',  -- pending | processing | done | partial | failed
+    created_at   TEXT    NOT NULL DEFAULT (datetime('now'))
+);
+
+CREATE TABLE IF NOT EXISTS social_targets (
+    id            INTEGER PRIMARY KEY AUTOINCREMENT,
+    post_id       INTEGER NOT NULL REFERENCES social_posts(id) ON DELETE CASCADE,
+    project_id    INTEGER NOT NULL,
+    platform      TEXT    NOT NULL,   -- discord | mastodon | linkedin
+    target_key    TEXT    NOT NULL,
+    include_image INTEGER NOT NULL DEFAULT 0,
+    status        TEXT    NOT NULL DEFAULT 'pending',  -- pending | sent | failed
+    error         TEXT,
+    sent_at       TEXT
+);
+
+CREATE INDEX IF NOT EXISTS idx_social_posts_status ON social_posts(status, scheduled_at);
+CREATE INDEX IF NOT EXISTS idx_social_targets_post  ON social_targets(post_id);