Eviction no longer reads a whole table while requests wait on the database (closes #227)

UsageBytes now reads the new cache_usage row, which triggers on
source_content and variant_content keep up to date in the statement
that adds, removes or resizes a row. The reconciliation pass reads both
tables 1000 rows per query, sums them, and corrects the total when it
differs, unless a row changed while it summed. Source rows now get
last_accessed_at when added, so choosing source images to evict reads
that column's index instead of sorting the whole table. Stats still
sums the tables, now in pages: an existing test drops both tables and
expects that sum to fail.

Model: opus-5-5

internal/db/migrations/001_schema.sql

@@ -3,14 +3,12 @@
 
 -- Source content blobs
 -- Files stored at: cache/sources/<ab>/<cd>/<sha256>
--- last_accessed_at is NULL until the first LRU touch; eviction falls
--- back to fetched_at for rows that have never been touched.
 CREATE TABLE IF NOT EXISTS source_content (
     content_hash TEXT PRIMARY KEY,
     content_type TEXT NOT NULL,
     size_bytes INTEGER NOT NULL,
     fetched_at DATETIME DEFAULT CURRENT_TIMESTAMP,
-    last_accessed_at DATETIME
+    last_accessed_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
 );
 CREATE INDEX IF NOT EXISTS idx_source_content_last_accessed
     ON source_content(last_accessed_at);
@@ -42,9 +40,8 @@ CREATE INDEX IF NOT EXISTS idx_source_meta_content_hash ON source_metadata(conte
 -- Processed variant blobs
 -- Files stored at: cache/variants/<ab>/<cd>/<cache_key> (plus a .meta
 -- sidecar with the content type). Tracked here (like source content
--- blobs above) so total cache usage can be computed with a SUM query,
--- never a directory scan, and so LRU eviction has a timestamp to order
--- on.
+-- blobs above) so total cache usage is known without a directory scan,
+-- and so LRU eviction has a timestamp to order on.
 CREATE TABLE IF NOT EXISTS variant_content (
     cache_key TEXT PRIMARY KEY,
     size_bytes INTEGER NOT NULL,
@@ -55,6 +52,74 @@ CREATE TABLE IF NOT EXISTS variant_content (
 CREATE INDEX IF NOT EXISTS idx_variant_content_last_accessed
     ON variant_content(last_accessed_at);
 
+-- Total cache usage: the sum of size_bytes over source_content and
+-- variant_content, kept by the triggers below in the same statement
+-- that adds, removes or resizes a row, so eviction reads this one row
+-- instead of summing both tables. Each trigger also adds one to
+-- change_count; the reconciliation pass, which sums both tables a page
+-- at a time, corrects total_size_bytes only if change_count did not
+-- move while it summed.
+CREATE TABLE IF NOT EXISTS cache_usage (
+    id INTEGER PRIMARY KEY CHECK (id = 1),
+    total_size_bytes INTEGER NOT NULL DEFAULT 0,
+    change_count INTEGER NOT NULL DEFAULT 0
+);
+INSERT OR IGNORE INTO cache_usage (id) VALUES (1);
+
+CREATE TRIGGER IF NOT EXISTS source_content_usage_insert
+AFTER INSERT ON source_content
+BEGIN
+    UPDATE cache_usage
+    SET total_size_bytes = total_size_bytes + NEW.size_bytes,
+        change_count = change_count + 1
+    WHERE id = 1;
+END;
+
+CREATE TRIGGER IF NOT EXISTS source_content_usage_delete
+AFTER DELETE ON source_content
+BEGIN
+    UPDATE cache_usage
+    SET total_size_bytes = total_size_bytes - OLD.size_bytes,
+        change_count = change_count + 1
+    WHERE id = 1;
+END;
+
+CREATE TRIGGER IF NOT EXISTS source_content_usage_update
+AFTER UPDATE OF size_bytes ON source_content
+BEGIN
+    UPDATE cache_usage
+    SET total_size_bytes = total_size_bytes - OLD.size_bytes + NEW.size_bytes,
+        change_count = change_count + 1
+    WHERE id = 1;
+END;
+
+CREATE TRIGGER IF NOT EXISTS variant_content_usage_insert
+AFTER INSERT ON variant_content
+BEGIN
+    UPDATE cache_usage
+    SET total_size_bytes = total_size_bytes + NEW.size_bytes,
+        change_count = change_count + 1
+    WHERE id = 1;
+END;
+
+CREATE TRIGGER IF NOT EXISTS variant_content_usage_delete
+AFTER DELETE ON variant_content
+BEGIN
+    UPDATE cache_usage
+    SET total_size_bytes = total_size_bytes - OLD.size_bytes,
+        change_count = change_count + 1
+    WHERE id = 1;
+END;
+
+CREATE TRIGGER IF NOT EXISTS variant_content_usage_update
+AFTER UPDATE OF size_bytes ON variant_content
+BEGIN
+    UPDATE cache_usage
+    SET total_size_bytes = total_size_bytes - OLD.size_bytes + NEW.size_bytes,
+        change_count = change_count + 1
+    WHERE id = 1;
+END;
+
 -- Output/transformed content blobs
 -- Not written: transformed images are stored in cache/variants and
 -- tracked in variant_content above.