feat: implement gallery feed with social features and SSE

- Cursor-based feed endpoint using v_feed view with hashtag filtering
- Like toggle (INSERT ON CONFLICT), comments CRUD
- Feed delta endpoint for SSE-driven incremental updates
- SSE client with Page Visibility API (pause/reconnect)
- Responsive photo/video grid with infinite scroll
- Hashtag filter chips, lightbox modal with comments
- Media file serving via tower-http ServeDir

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/handlers/social.rs

@@ -0,0 +1,136 @@
+use axum::extract::{Path, State};
+use axum::http::StatusCode;
+use axum::Json;
+use serde::Deserialize;
+use uuid::Uuid;
+
+use crate::auth::middleware::AuthUser;
+use crate::error::AppError;
+use crate::models::comment::{Comment, CommentDto};
+use crate::models::hashtag::{self, Hashtag};
+use crate::state::AppState;
+
+pub async fn toggle_like(
+    State(state): State<AppState>,
+    auth: AuthUser,
+    Path(upload_id): Path<Uuid>,
+) -> Result<StatusCode, AppError> {
+    // Check if user is banned
+    let user = crate::models::user::User::find_by_id(&state.pool, auth.user_id)
+        .await?
+        .ok_or_else(|| AppError::NotFound("Benutzer nicht gefunden.".into()))?;
+    if user.is_banned {
+        return Err(AppError::Forbidden("Du bist gesperrt.".into()));
+    }
+
+    // Try to insert; if conflict, delete (toggle)
+    let result = sqlx::query(
+        "INSERT INTO \"like\" (upload_id, user_id) VALUES ($1, $2)
+         ON CONFLICT (upload_id, user_id) DO NOTHING",
+    )
+    .bind(upload_id)
+    .bind(auth.user_id)
+    .execute(&state.pool)
+    .await?;
+
+    if result.rows_affected() == 0 {
+        // Already liked — remove
+        sqlx::query("DELETE FROM \"like\" WHERE upload_id = $1 AND user_id = $2")
+            .bind(upload_id)
+            .bind(auth.user_id)
+            .execute(&state.pool)
+            .await?;
+    }
+
+    // Broadcast SSE
+    let _ = state.sse_tx.send(crate::state::SseEvent {
+        event_type: "like-update".to_string(),
+        data: serde_json::json!({ "upload_id": upload_id }).to_string(),
+    });
+
+    Ok(StatusCode::NO_CONTENT)
+}
+
+pub async fn list_comments(
+    State(state): State<AppState>,
+    _auth: AuthUser,
+    Path(upload_id): Path<Uuid>,
+) -> Result<Json<Vec<CommentDto>>, AppError> {
+    let comments = Comment::list_for_upload(&state.pool, upload_id).await?;
+    Ok(Json(comments))
+}
+
+#[derive(Deserialize)]
+pub struct AddCommentRequest {
+    pub body: String,
+}
+
+pub async fn add_comment(
+    State(state): State<AppState>,
+    auth: AuthUser,
+    Path(upload_id): Path<Uuid>,
+    Json(body): Json<AddCommentRequest>,
+) -> Result<(StatusCode, Json<CommentDto>), AppError> {
+    let user = crate::models::user::User::find_by_id(&state.pool, auth.user_id)
+        .await?
+        .ok_or_else(|| AppError::NotFound("Benutzer nicht gefunden.".into()))?;
+    if user.is_banned {
+        return Err(AppError::Forbidden("Du bist gesperrt.".into()));
+    }
+
+    let text = body.body.trim();
+    if text.is_empty() || text.len() > 500 {
+        return Err(AppError::BadRequest(
+            "Kommentar muss zwischen 1 und 500 Zeichen lang sein.".into(),
+        ));
+    }
+
+    let comment = Comment::create(&state.pool, upload_id, auth.user_id, text).await?;
+
+    // Process hashtags in comment body
+    let tags = hashtag::extract_hashtags(text);
+    for tag in &tags {
+        let h = Hashtag::upsert(&state.pool, auth.event_id, tag).await?;
+        sqlx::query(
+            "INSERT INTO comment_hashtag (comment_id, hashtag_id) VALUES ($1, $2) ON CONFLICT DO NOTHING",
+        )
+        .bind(comment.id)
+        .bind(h.id)
+        .execute(&state.pool)
+        .await?;
+    }
+
+    // Broadcast SSE
+    let _ = state.sse_tx.send(crate::state::SseEvent {
+        event_type: "new-comment".to_string(),
+        data: serde_json::json!({ "upload_id": upload_id }).to_string(),
+    });
+
+    let dto = CommentDto {
+        id: comment.id,
+        upload_id,
+        user_id: auth.user_id,
+        uploader_name: user.display_name,
+        body: comment.body,
+        created_at: comment.created_at,
+    };
+
+    Ok((StatusCode::CREATED, Json(dto)))
+}
+
+pub async fn delete_comment(
+    State(state): State<AppState>,
+    auth: AuthUser,
+    Path(comment_id): Path<Uuid>,
+) -> Result<StatusCode, AppError> {
+    let comment = Comment::find_by_id(&state.pool, comment_id)
+        .await?
+        .ok_or_else(|| AppError::NotFound("Kommentar nicht gefunden.".into()))?;
+
+    if comment.user_id != auth.user_id {
+        return Err(AppError::Forbidden("Nur eigene Kommentare löschen.".into()));
+    }
+
+    Comment::soft_delete(&state.pool, comment_id).await?;
+    Ok(StatusCode::NO_CONTENT)
+}