feat(series): add SQL queries for series browsing and continue-series

Add five new sqlc queries to support the series browse page and
continue-series dashboard collection:

- GetDistinctSeries: list unique series with book counts, sorted by
  most recent entry, with pagination
- GetDistinctSeriesCount: total distinct series count for pagination
- GetSeriesCovers: fetch up to N cover image paths for a series,
  ordered by series_number
- GetSeriesBooks: fetch all books in a series ordered by series_number
- GetContinueSeriesItems: CTE-based query using DISTINCT ON to find
  the next unread book per series for a given user/library, sorted
  by most recent last_read_at

internal/database/queries/queries.sql

@@ -1905,6 +1905,59 @@ SELECT mi.* FROM media_items mi
 WHERE mi.library_id = $1
 ORDER BY mi.created_at DESC;
 
+-- name: GetDistinctSeries :many
+SELECT series, COUNT(*) as book_count,
+       MAX(series_count) as total_in_series,
+       MAX(created_at) as last_entry_at
+FROM media_items
+WHERE library_id = $1 AND series IS NOT NULL AND series != ''
+GROUP BY series
+ORDER BY MAX(created_at) DESC
+LIMIT $2 OFFSET $3;
+
+-- name: GetDistinctSeriesCount :one
+SELECT COUNT(DISTINCT series)::int
+FROM media_items
+WHERE library_id = $1 AND series IS NOT NULL AND series != '';
+
+-- name: GetSeriesCovers :many
+SELECT cover_image_path, library_id
+FROM media_items
+WHERE library_id = $1 AND series = $2 AND cover_image_path IS NOT NULL AND cover_image_path != ''
+ORDER BY series_number ASC NULLS LAST
+LIMIT $3;
+
+-- name: GetSeriesBooks :many
+SELECT * FROM media_items
+WHERE library_id = $1 AND series = $2
+ORDER BY series_number ASC NULLS LAST;
+
+-- name: GetContinueSeriesItems :many
+WITH user_series_progress AS (
+    SELECT mi.series,
+           MAX(mi.series_number) as max_read_number,
+           MAX(rp.last_read_at) as last_read_at
+    FROM reading_progress rp
+    JOIN media_items mi ON mi.id = rp.media_item_id
+    WHERE rp.user_id = $2
+      AND rp.percentage > 0
+      AND mi.series IS NOT NULL AND mi.series != ''
+      AND mi.library_id = $1
+    GROUP BY mi.series
+),
+next_books AS (
+    SELECT DISTINCT ON (mi.series) mi.*,
+           usp.last_read_at
+    FROM media_items mi
+    JOIN user_series_progress usp ON mi.series = usp.series
+    WHERE mi.library_id = $1
+      AND (mi.series_number > usp.max_read_number OR usp.max_read_number IS NULL)
+    ORDER BY mi.series, mi.series_number ASC NULLS LAST
+)
+SELECT * FROM next_books
+ORDER BY last_read_at DESC NULLS LAST
+LIMIT $3;
+
 -- name: GetSavedFilters :many
 SELECT * FROM saved_filters
 WHERE user_id = @user_id AND resource_type = @resource_type