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>

Caddyfile

@@ -1,26 +1,38 @@
 {$DOMAIN} {
-    encode zstd gzip
+	encode zstd gzip
 
-    # SvelteKit frontend — static assets with long-lived cache (content-hashed filenames)
-    @hashed_assets path_regexp hashed /_app/immutable/.*\.[a-f0-9]{8,}\.(js|css|woff2)$
-    header @hashed_assets Cache-Control "public, max-age=31536000, immutable"
+	# Site-wide security headers (defense-in-depth). HSTS is free since Caddy
+	# already terminates TLS. nosniff also covers all of /media/*.
+	header {
+		Strict-Transport-Security "max-age=31536000; includeSubDomains"
+		X-Content-Type-Options "nosniff"
+		X-Frame-Options "DENY"
+		Referrer-Policy "strict-origin-when-cross-origin"
+	}
 
-    # Media previews and thumbnails
-    @previews path /media/previews/* /media/thumbnails/*
-    header @previews Cache-Control "public, max-age=3600"
+	# SvelteKit frontend — static assets with long-lived cache (content-hashed filenames)
+	@hashed_assets path_regexp hashed /_app/immutable/.*\.[a-f0-9]{8,}\.(js|css|woff2)$
+	header @hashed_assets Cache-Control "public, max-age=31536000, immutable"
 
-    # Original media files (private — only host can download)
-    @originals path /media/originals/*
-    header @originals Cache-Control "private, max-age=86400"
+	# Media previews and thumbnails
+	@previews path /media/previews/* /media/thumbnails/*
+	header @previews Cache-Control "public, max-age=3600"
 
-    # API — never cache
-    @api path /api/*
-    header @api Cache-Control "no-store"
+	# Original media files (private — only host can download). Force download
+	# rather than inline rendering as defense-in-depth against any future
+	# content-type confusion (previews/thumbnails are re-encoded and stay inline).
+	@originals path /media/originals/*
+	header @originals Cache-Control "private, max-age=86400"
+	header @originals Content-Disposition "attachment"
 
-    # Route API and media requests to the Rust backend
-    reverse_proxy /api/* app:3000
-    reverse_proxy /media/* app:3000
+	# API — never cache
+	@api path /api/*
+	header @api Cache-Control "no-store"
 
-    # Everything else goes to SvelteKit frontend
-    reverse_proxy frontend:3001
+	# Route API and media requests to the Rust backend
+	reverse_proxy /api/* app:3000
+	reverse_proxy /media/* app:3000
+
+	# Everything else goes to SvelteKit frontend
+	reverse_proxy frontend:3001
 }