perf(feed): rewrite v_feed, bound feed_delta, server-time SSE cursor (H6,H7,M9)

H6: migration 006 replaces v_feed's double LEFT JOIN + COUNT(DISTINCT) (which
materialized a likes×comments Cartesian per upload) with correlated scalar
subqueries that each use their own index. Output columns are unchanged, so
feed + hashtag-filtered paths both benefit.

H7: feed_delta now applies the feed rate limit (keyed per user), caps results
at 200 rows, and clamps how far back a client `since` may reach (7 days). When
clamped or capped it returns reload_required=true; the SSE client turns that
into a full feed reload instead of streaming the whole gallery through the view
on every tab refocus.

M9: the SSE reconnect cursor is now advanced only from server timestamps (an
upload's created_at, seeded from the feed and updated on new-upload events and
delta responses), never the client clock — so a skewed phone clock can't drop
events missed while backgrounded.

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

backend/migrations/006_feed_view_perf.down.sql

@@ -0,0 +1,23 @@
+-- Restore the original join-based v_feed definition.
+CREATE OR REPLACE VIEW v_feed AS
+SELECT
+    u.id,
+    u.event_id,
+    u.user_id,
+    usr.display_name             AS uploader_name,
+    usr.is_banned,
+    usr.uploads_hidden,
+    u.preview_path,
+    u.thumbnail_path,
+    u.mime_type,
+    u.caption,
+    u.created_at,
+    COUNT(DISTINCT l.user_id)    AS like_count,
+    COUNT(DISTINCT c.id)         AS comment_count
+FROM upload u
+JOIN "user" usr ON u.user_id = usr.id
+LEFT JOIN "like" l  ON l.upload_id = u.id
+LEFT JOIN comment c ON c.upload_id = u.id AND c.deleted_at IS NULL
+WHERE u.deleted_at IS NULL
+  AND usr.uploads_hidden = FALSE
+GROUP BY u.id, usr.display_name, usr.is_banned, usr.uploads_hidden;

backend/migrations/006_feed_view_perf.up.sql

@@ -0,0 +1,26 @@
+-- H6: replace v_feed's double LEFT JOIN + COUNT(DISTINCT) (which materializes a
+-- likes×comments Cartesian per upload before de-duping) with correlated scalar
+-- subqueries. Each count now uses its own index (idx_like_upload /
+-- idx_comment_upload) and there is no GROUP BY. The output columns are
+-- unchanged, so every consumer keeps working.
+CREATE OR REPLACE VIEW v_feed AS
+SELECT
+    u.id,
+    u.event_id,
+    u.user_id,
+    usr.display_name             AS uploader_name,
+    usr.is_banned,
+    usr.uploads_hidden,
+    u.preview_path,
+    u.thumbnail_path,
+    u.mime_type,
+    u.caption,
+    u.created_at,
+    (SELECT COUNT(*) FROM "like" l
+       WHERE l.upload_id = u.id)                              AS like_count,
+    (SELECT COUNT(*) FROM comment c
+       WHERE c.upload_id = u.id AND c.deleted_at IS NULL)     AS comment_count
+FROM upload u
+JOIN "user" usr ON u.user_id = usr.id
+WHERE u.deleted_at IS NULL
+  AND usr.uploads_hidden = FALSE;