feat: migrate from SQLite to PostgreSQL - complete schema and API updates

- Remove better-sqlite3, add pg driver
- Convert schema to PostgreSQL types (serial, timestamp, boolean, jsonb)
- Generate fresh PostgreSQL migrations
- Update database connection layer with pg.Pool
- Fix all API routes: remove JSON.parse/stringify, use native timestamps and booleans
- Update drizzle.config.ts and .env.example for PostgreSQL

drizzle/meta/_journal.json

@@ -1,47 +1,12 @@
 {
   "version": "7",
-  "dialect": "sqlite",
+  "dialect": "postgresql",
   "entries": [
     {
       "idx": 0,
-      "version": "6",
-      "when": 1770907611962,
-      "tag": "0000_goofy_captain_midlands",
-      "breakpoints": true
-    },
-    {
-      "idx": 1,
-      "version": "6",
-      "when": 1770915891699,
-      "tag": "0001_sticky_shinko_yamashiro",
-      "breakpoints": true
-    },
-    {
-      "idx": 2,
-      "version": "6",
-      "when": 1770937855462,
-      "tag": "0002_careful_synch",
-      "breakpoints": true
-    },
-    {
-      "idx": 3,
-      "version": "6",
-      "when": 1771044740273,
-      "tag": "0003_demonic_captain_flint",
-      "breakpoints": true
-    },
-    {
-      "idx": 4,
-      "version": "6",
-      "when": 1771055000000,
-      "tag": "0004_add_default_span_label_types",
-      "breakpoints": true
-    },
-    {
-      "idx": 5,
-      "version": "6",
-      "when": 1771156000000,
-      "tag": "0005_add_source_and_model_prediction_to_span_annotations",
+      "version": "7",
+      "when": 1771332001387,
+      "tag": "0000_nifty_gauntlet",
       "breakpoints": true
     }
   ]