fix: fold cache eviction schema into 001_initial_schema.sql

Pre-1.0 with no installed base to migrate: REPO_POLICIES.md forbids
adding numbered migration files beyond 001 before a tagged release.
Fold the variant_content table and source_content.last_accessed_at
column (previously 002_cache_eviction.sql) directly into
001_initial_schema.sql and delete the 002 file. The migration runner
is generic over whatever *.sql files exist in schema/, so no runner
code changes are needed.

internal/database/schema/001_initial_schema.sql

@@ -3,12 +3,17 @@
 
 -- Source content blobs
 -- Files stored at: cache/src-content/<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
+    fetched_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    last_accessed_at DATETIME
 );
+CREATE INDEX IF NOT EXISTS idx_source_content_last_accessed
+    ON source_content(last_accessed_at);
 
 -- Source URL metadata - maps URLs to content hashes
 -- JSON stored at: cache/src-metadata/<hostname>/<path_hash>.json
@@ -34,6 +39,22 @@ CREATE INDEX IF NOT EXISTS idx_source_meta_path_hash ON source_metadata(path_has
 CREATE INDEX IF NOT EXISTS idx_source_meta_expires ON source_metadata(expires_at);
 CREATE INDEX IF NOT EXISTS idx_source_meta_content_hash ON source_metadata(content_hash);
 
+-- 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.
+CREATE TABLE IF NOT EXISTS variant_content (
+    cache_key TEXT PRIMARY KEY,
+    size_bytes INTEGER NOT NULL,
+    content_type TEXT NOT NULL,
+    created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    last_accessed_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+CREATE INDEX IF NOT EXISTS idx_variant_content_last_accessed
+    ON variant_content(last_accessed_at);
+
 -- Output/transformed content blobs
 -- Files stored at: cache/dst-content/<ab>/<cd>/<sha256>
 CREATE TABLE IF NOT EXISTS output_content (

internal/database/schema/002_cache_eviction.sql

@@ -1,25 +0,0 @@
--- Migration 002: cache size accounting and eviction
---
--- Tracks processed variants in the database (source content blobs are
--- already tracked in source_content) so total cache usage can be
--- computed without directory scans, and adds last-access timestamps
--- for LRU eviction ordering.
-
--- Processed variant blobs
--- Files stored at: cache/variants/<ab>/<cd>/<cache_key> (plus a
--- .meta sidecar with the content type)
-CREATE TABLE IF NOT EXISTS variant_content (
-    cache_key TEXT PRIMARY KEY,
-    size_bytes INTEGER NOT NULL,
-    content_type TEXT NOT NULL,
-    created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
-    last_accessed_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
-);
-CREATE INDEX IF NOT EXISTS idx_variant_content_last_accessed
-    ON variant_content(last_accessed_at);
-
--- LRU timestamp for source content blobs. Rows written before this
--- migration have NULL here; eviction falls back to fetched_at.
-ALTER TABLE source_content ADD COLUMN last_accessed_at DATETIME;
-CREATE INDEX IF NOT EXISTS idx_source_content_last_accessed
-    ON source_content(last_accessed_at);