fix(security): batch-2 hardening — XSS allowlist, event-scoping, deploy

Address the seven findings from the security & deployment review.

High:
- upload: make infer authoritative — reject files it can't identify
  (SVG/HTML/JS) and require the detected MIME to be on an ALLOWED_MEDIA
  allowlist; derive the stored MIME and on-disk extension from the
  detected type, ignoring client filename/Content-Type. Closes the
  stored-XSS vector via media served on-origin.
- deploy: rename docker-compose.override.yml -> docker-compose.dev.yml
  so the default `docker compose up -d` no longer publishes Postgres
  5432 to the host; the port map is now opt-in via -f. README updated.

Medium:
- upload: DefaultBodyLimit::disable() -> max(576 MiB) as an HTTP-level
  OOM backstop; handler still enforces precise per-class size limits.
- docker: run backend and frontend as non-root users.

Low:
- social/upload: event-scope toggle_like, list_comments, add_comment,
  delete_comment, edit_upload, delete_upload via find_by_id_and_event /
  soft_delete_in_event — cross-event IDs now resolve to 404.
- Caddy: site-wide HSTS / nosniff / X-Frame-Options / Referrer-Policy,
  plus Content-Disposition: attachment on /media/originals/*.
- .env.example: replace default Postgres password with a CHANGE_ME hint.

Out of scope: localStorage JWT (root cause fixed; httpOnly cookies are a
larger change tracked separately).

Verified: cargo build (no new warnings), cargo test (3 passed),
caddy validate, docker compose config (no 5432 published by default).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/main.rs

@@ -18,6 +18,10 @@ mod state;
 use config::AppConfig;
 use state::AppState;
 
+/// Hard HTTP body cap for the upload endpoint (576 MiB). Backstop against
+/// memory-exhaustion; precise per-class size limits are enforced in the handler.
+const MAX_UPLOAD_BYTES: usize = 576 * 1024 * 1024;
+
 #[tokio::main]
 async fn main() -> Result<()> {
     dotenvy::dotenv().ok();
@@ -57,9 +61,13 @@ async fn main() -> Result<()> {
         .route("/api/v1/recover", post(auth::handlers::recover))
         .route("/api/v1/admin/login", post(auth::handlers::admin_login))
         .route("/api/v1/session", delete(auth::handlers::logout))
-        // Upload — body limit disabled; size validation is done inside the handler
+        // Upload — HTTP-level body cap as an OOM backstop. The handler still enforces
+        // the precise per-class limits from DB config (max_image/video_size_mb); this
+        // layer just stops a multi-GB body from being buffered into memory before that
+        // check runs. Sized generously above the default 500 MB video limit + multipart
+        // overhead — if an admin raises max_video_size_mb above this, bump MAX_UPLOAD_BYTES.
         .route("/api/v1/upload", post(handlers::upload::upload)
-            .route_layer(DefaultBodyLimit::disable()))
+            .route_layer(DefaultBodyLimit::max(MAX_UPLOAD_BYTES)))
         .route(
             "/api/v1/upload/{id}",
             patch(handlers::upload::edit_upload).delete(handlers::upload::delete_upload),