fix(config): auto-route test.db locally, posterg.db on production

- config.php: getDatabasePath() detects php built-in CLI server
  (php_sapi_name() === 'cli-server') and routes to test.db; all
  other SAPIs (nginx/fpm) get posterg.db. DB_ENV env-var still
  overrides either way.

- migrate.sh: auto-initialise the target DB from storage/schema.sql
  when the file is absent or has no tables yet. Existing DBs with
  data are left completely untouched (table_count check, no re-run
  of schema on populated DB). Idempotent: safe to run repeatedly.

- justfile: serve still calls migrate (which now handles init too),
  no DB_ENV prefix needed since sapi detection handles routing.

src/config.php

@@ -8,27 +8,33 @@
 // Database paths relative to repository root
 define('DB_ROOT', __DIR__ . '/..');
 
-// Test database (used in development)
+// Local dev database (php -S, never committed)
 define('DB_TEST_PATH', DB_ROOT . '/storage/test.db');
 
-// Production database (used on server)
+// Production database (nginx/php-fpm on server)
 define('DB_PROD_PATH', DB_ROOT . '/storage/posterg.db');
 
 /**
  * Determine which database to use.
  *
- * Always defaults to the production database.
- * Set DB_ENV=test explicitly to use the test database.
+ * - php built-in server (php -S …)  → storage/test.db   (local dev)
+ * - everything else (nginx/fpm)     → storage/posterg.db (production)
  *
- *   export DB_ENV=test   # use storage/test.db
- *   export DB_ENV=prod   # use storage/posterg.db (default)
- *
- * The old file-existence auto-detection has been removed: a leftover
- * test.db on a developer machine no longer silently redirects all
- * requests to test data.
+ * The DB_ENV env-var can still override either way:
+ *   DB_ENV=test  → force test.db
+ *   DB_ENV=prod  → force posterg.db
  */
 function getDatabasePath(): string {
-    if (getenv('DB_ENV') === 'test') {
+    $env = getenv('DB_ENV');
+    if ($env === 'test') {
+        return DB_TEST_PATH;
+    }
+    if ($env === 'prod') {
+        return DB_PROD_PATH;
+    }
+
+    // Auto-detect: php built-in CLI server == local development
+    if (php_sapi_name() === 'cli-server') {
         return DB_TEST_PATH;
     }
 
@@ -36,8 +42,8 @@ function getDatabasePath(): string {
 }
 
 /**
- * Check if running in test/development mode
+ * Check if running in local dev mode
  */
-function isTestMode() {
+function isTestMode(): bool {
     return getDatabasePath() === DB_TEST_PATH;
 }