fix: auto-reinit PG cluster when extensions or version change

- Write pg derivation path to .nix-postgres-version after init
- On start, compare current pg path with stored version
- If changed (extensions added/removed/version bumped), destroy and
  reinitialize the cluster automatically
- Also reinit legacy clusters that have no version file
- Exclude .nix-postgres-version from release builds

nix/build.nix

@@ -33,7 +33,8 @@ beamPackages.mixRelease {
       && baseName != ".nix-mix"
       && baseName != ".nix-hex"
       && baseName != ".worktrees"
-      && baseName != ".nix-services-started";
+      && baseName != ".nix-services-started"
+      && baseName != ".nix-postgres-version";
   };
 
   # Mix environment

nix/shell.nix

@@ -44,6 +44,9 @@ let
   # Service management flag
   servicesFlag = ".nix-services-started";
 
+  # Version file to detect when PG config changes (extensions added/removed, version bump)
+  pgVersionFile = ".nix-postgres-version";
+
   # Script to start PostgreSQL and Redis
   startServices = writeShellScriptBin "start-services" ''
     set -e
@@ -56,6 +59,27 @@ let
       exit 0
     fi
 
+    # Check if cluster needs reinitialization (PG extensions or version changed)
+    pg_needs_reinit=false
+    if [ -d "${pgDataDir}" ] && [ -f "${pgVersionFile}" ]; then
+      if [ "$(cat ${pgVersionFile})" != "${pg}" ]; then
+        echo "⚠️  PostgreSQL configuration changed (extensions or version updated)"
+        pg_needs_reinit=true
+      fi
+    fi
+    if [ -d "${pgDataDir}" ] && [ ! -f "${pgVersionFile}" ]; then
+      # Legacy cluster without version file, reinit to be safe
+      pg_needs_reinit=true
+    fi
+
+    if $pg_needs_reinit; then
+      echo "🗑️  Removing old PostgreSQL cluster..."
+      # Stop if running
+      ${pg}/bin/pg_ctl -D ${pgDataDir} stop -m fast 2>/dev/null || true
+      rm -rf ${pgDataDir}
+      rm -f ${servicesFlag}
+    fi
+
     # Initialize PostgreSQL if needed
     if [ ! -d "${pgDataDir}" ]; then
       echo "📦 Initializing PostgreSQL database..."
@@ -73,6 +97,9 @@ let
     shared_preload_libraries = 'timescaledb'
     timescaledb.telemetry_level = off
     EOF
+
+      # Record current PG version for future change detection
+      echo -n "${pg}" > ${pgVersionFile}
     fi
 
     # Check if PostgreSQL is already running on this port