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/handlers/social.rs

@@ -9,6 +9,7 @@ use crate::auth::middleware::AuthUser;
 use crate::error::AppError;
 use crate::models::comment::{Comment, CommentDto};
 use crate::models::hashtag::{self, Hashtag};
+use crate::models::upload::Upload;
 use crate::state::AppState;
 
 pub async fn toggle_like(
@@ -24,6 +25,12 @@ pub async fn toggle_like(
         return Err(AppError::Forbidden("Du bist gesperrt.".into()));
     }
 
+    // Event-scope: the upload must belong to the caller's event (404 otherwise),
+    // matching the host handlers' find_by_id_and_event pattern.
+    Upload::find_by_id_and_event(&state.pool, upload_id, auth.event_id)
+        .await?
+        .ok_or_else(|| AppError::NotFound("Upload nicht gefunden.".into()))?;
+
     // Try to insert; if conflict, delete (toggle)
     let result = sqlx::query(
         "INSERT INTO \"like\" (upload_id, user_id) VALUES ($1, $2)
@@ -64,10 +71,15 @@ const COMMENT_PAGE_SIZE: i64 = 50;
 
 pub async fn list_comments(
     State(state): State<AppState>,
-    _auth: AuthUser,
+    auth: AuthUser,
     Path(upload_id): Path<Uuid>,
     Query(q): Query<ListCommentsQuery>,
 ) -> Result<Json<Vec<CommentDto>>, AppError> {
+    // Event-scope: only list comments for an upload in the caller's event.
+    Upload::find_by_id_and_event(&state.pool, upload_id, auth.event_id)
+        .await?
+        .ok_or_else(|| AppError::NotFound("Upload nicht gefunden.".into()))?;
+
     let comments =
         Comment::list_for_upload(&state.pool, upload_id, q.before, COMMENT_PAGE_SIZE).await?;
     Ok(Json(comments))
@@ -91,6 +103,11 @@ pub async fn add_comment(
         return Err(AppError::Forbidden("Du bist gesperrt.".into()));
     }
 
+    // Event-scope: only comment on an upload that belongs to the caller's event.
+    Upload::find_by_id_and_event(&state.pool, upload_id, auth.event_id)
+        .await?
+        .ok_or_else(|| AppError::NotFound("Upload nicht gefunden.".into()))?;
+
     let text = body.body.trim();
     let text_chars = text.chars().count();
     if text_chars == 0 || text_chars > 500 {
@@ -145,6 +162,11 @@ pub async fn delete_comment(
         return Err(AppError::Forbidden("Nur eigene Kommentare löschen.".into()));
     }
 
-    Comment::soft_delete(&state.pool, comment_id).await?;
+    // Event-scope: soft_delete_in_event only matches comments whose upload is in
+    // the caller's event, so a cross-event comment_id resolves to a 404 here.
+    let deleted = Comment::soft_delete_in_event(&state.pool, comment_id, auth.event_id).await?;
+    if !deleted {
+        return Err(AppError::NotFound("Kommentar nicht gefunden.".into()));
+    }
     Ok(StatusCode::NO_CONTENT)
 }