feat: add admin publishing workflow and yar theme

Add Go/Postgres admin APIs, Angular admin UI, manual build flow, asset uploads, markdown import/export, configurable slug generation, and the Yar reading theme. Exclude local docs and generated development artifacts from version control.

backend/migrations/002_admin_sessions.sql

@@ -0,0 +1,11 @@
+CREATE TABLE IF NOT EXISTS admin_sessions (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    token_hash TEXT NOT NULL UNIQUE,
+    expires_at TIMESTAMPTZ NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    last_seen_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS idx_admin_sessions_user_id ON admin_sessions(user_id);
+CREATE INDEX IF NOT EXISTS idx_admin_sessions_expires_at ON admin_sessions(expires_at);