Count hits, misses, fetches and transforms as Prometheus counters (closes #311)

The five counts the cache_stats table held are now Prometheus counters in
memory, as fields of the cache. NewCache makes them and registers them on
the registry it is given. pixad gives it Prometheus' default registry,
which /metrics serves; each test gives it a new one. The Increment methods
add to their cache's counters, so counting makes no database call.

cache_stats, Cache.Stats, Service.Stats, CacheStats and the pending counts
are gone, with their tests. The background goroutine now writes only access
times, and its names say so: StartPendingAccessTimeWrites,
StopPendingAccessTimeWrites and pendingaccesstimes.go. The two stop tests
whose write under way was a count now use access times.

Model: opus-5-5

internal/db/migrations/001_schema.sql

@@ -108,18 +108,6 @@ CREATE TABLE IF NOT EXISTS negative_cache (
 );
 CREATE INDEX IF NOT EXISTS idx_negative_cache_expires ON negative_cache(expires_at);
 
--- Cache statistics for monitoring
-CREATE TABLE IF NOT EXISTS cache_stats (
-    id INTEGER PRIMARY KEY CHECK (id = 1),
-    hit_count INTEGER NOT NULL DEFAULT 0,
-    miss_count INTEGER NOT NULL DEFAULT 0,
-    upstream_fetch_count INTEGER NOT NULL DEFAULT 0,
-    upstream_fetch_bytes INTEGER NOT NULL DEFAULT 0,
-    transform_count INTEGER NOT NULL DEFAULT 0,
-    last_updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
-);
-INSERT OR IGNORE INTO cache_stats (id) VALUES (1);
-
 -- A random value that a login session carries from its login. pixa makes it
 -- when the table has no row, and each logout replaces it; a login session
 -- carrying another value is refused, so logging out ends every login session.