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/models/comment.rs

@@ -0,0 +1,72 @@
+use chrono::{DateTime, Utc};
+use serde::Serialize;
+use sqlx::PgPool;
+use uuid::Uuid;
+
+#[derive(Debug, sqlx::FromRow)]
+pub struct Comment {
+    pub id: Uuid,
+    pub upload_id: Uuid,
+    pub user_id: Uuid,
+    pub body: String,
+    pub created_at: DateTime<Utc>,
+    pub deleted_at: Option<DateTime<Utc>>,
+}
+
+#[derive(Debug, Serialize, sqlx::FromRow)]
+pub struct CommentDto {
+    pub id: Uuid,
+    pub upload_id: Uuid,
+    pub user_id: Uuid,
+    pub uploader_name: String,
+    pub body: String,
+    pub created_at: DateTime<Utc>,
+}
+
+impl Comment {
+    pub async fn create(
+        pool: &PgPool,
+        upload_id: Uuid,
+        user_id: Uuid,
+        body: &str,
+    ) -> Result<Self, sqlx::Error> {
+        sqlx::query_as::<_, Self>(
+            "INSERT INTO comment (upload_id, user_id, body) VALUES ($1, $2, $3) RETURNING *",
+        )
+        .bind(upload_id)
+        .bind(user_id)
+        .bind(body)
+        .fetch_one(pool)
+        .await
+    }
+
+    pub async fn list_for_upload(pool: &PgPool, upload_id: Uuid) -> Result<Vec<CommentDto>, sqlx::Error> {
+        sqlx::query_as::<_, CommentDto>(
+            "SELECT c.id, c.upload_id, c.user_id, u.display_name AS uploader_name, c.body, c.created_at
+             FROM comment c
+             JOIN \"user\" u ON u.id = c.user_id
+             WHERE c.upload_id = $1 AND c.deleted_at IS NULL
+             ORDER BY c.created_at ASC",
+        )
+        .bind(upload_id)
+        .fetch_all(pool)
+        .await
+    }
+
+    pub async fn find_by_id(pool: &PgPool, id: Uuid) -> Result<Option<Self>, sqlx::Error> {
+        sqlx::query_as::<_, Self>(
+            "SELECT * FROM comment WHERE id = $1 AND deleted_at IS NULL",
+        )
+        .bind(id)
+        .fetch_optional(pool)
+        .await
+    }
+
+    pub async fn soft_delete(pool: &PgPool, id: Uuid) -> Result<(), sqlx::Error> {
+        sqlx::query("UPDATE comment SET deleted_at = NOW() WHERE id = $1")
+            .bind(id)
+            .execute(pool)
+            .await?;
+        Ok(())
+    }
+}