Improve plugin source modeling and runtime dedup (#414)

crates/yaak-models/migrations/20260301000000_plugin-source-and-unique-directory.sql

@@ -0,0 +1,33 @@
+ALTER TABLE plugins
+    ADD COLUMN source TEXT DEFAULT 'filesystem' NOT NULL;
+
+-- Existing registry installs have a URL; classify them first.
+UPDATE plugins
+SET source = 'registry'
+WHERE url IS NOT NULL;
+
+-- Best-effort bundled backfill for legacy rows.
+UPDATE plugins
+SET source = 'bundled'
+WHERE source = 'filesystem'
+  AND (
+    -- Normalize separators so this also works for Windows paths.
+    replace(directory, '\', '/') LIKE '%/vendored/plugins/%'
+        OR replace(directory, '\', '/') LIKE '%/vendored-plugins/%'
+    );
+
+-- Keep one row per exact directory before adding uniqueness.
+-- Tie-break by recency.
+WITH ranked AS (SELECT id,
+                       ROW_NUMBER() OVER (
+                           PARTITION BY directory
+                           ORDER BY updated_at DESC,
+                               created_at DESC
+                           ) AS row_num
+                FROM plugins)
+DELETE
+FROM plugins
+WHERE id IN (SELECT id FROM ranked WHERE row_num > 1);
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_plugins_directory_unique
+    ON plugins (directory);