chore: add docker infrastructure, gists feature, attachment system, and admin CLI tools

- Add .dockerignore, Dockerfile, and docker compose targets to Makefile for containerized deployment
- Implement gists router with CRUD operations, database schema, and polymorphic comment/vote reuse
- Create attachment upload system with thumbnail generation, MIME detection, and storage path management
- Add attachments_prune CLI command to clean orphaned attachment records and files
- Introduce rate limiting middleware with 60 requests per minute window
- Add custom 404 and 500 error handlers with SEO-optimized template responses
- Extend database initialization with gists and attachments indexes plus upload site settings defaults
- Update load_comments to include attachment mapping for comment resources
- Register gists and uploads routers in main application and update AGENTS.md documentation

devplacepy/static/css/mention.css

@@ -0,0 +1,42 @@
+.mention-wrapper {
+    position: relative;
+    width: 100%;
+}
+
+.mention-dropdown {
+    display: none;
+    position: absolute;
+    bottom: 100%;
+    left: 0;
+    right: 0;
+    background: var(--bg-secondary);
+    border: 1px solid var(--border);
+    border-radius: var(--radius);
+    box-shadow: 0 -4px 12px rgba(0, 0, 0, 0.3);
+    z-index: 2100;
+    max-height: 200px;
+    overflow-y: auto;
+    margin-bottom: 4px;
+}
+
+.mention-dropdown-item {
+    display: flex;
+    align-items: center;
+    gap: 0.625rem;
+    width: 100%;
+    padding: 0.5rem 0.75rem;
+    font-size: 0.875rem;
+    color: var(--text-primary);
+    background: none;
+    border: none;
+    cursor: pointer;
+    text-align: left;
+    font-family: inherit;
+    transition: background 0.1s;
+}
+
+.mention-dropdown-item:hover,
+.mention-dropdown-item.active {
+    background: var(--accent-light);
+    color: var(--accent);
+}