perf(build): enable incremental tsc (#827)

backend/tsconfig.json had no incremental setting, so every tsc run
re-checked the full project from cold. The two frontend tsconfigs
already declared a tsBuildInfoFile path under node_modules/.tmp/ but
without incremental: true the file was never written, and the path
itself sits inside node_modules where npm ci wipes it on every fresh
install — neither of which actually persists incremental state.

Add incremental: true to all three configs and drop the broken
tsBuildInfoFile overrides. TypeScript's default places the buildinfo
next to the tsconfig (e.g. backend/tsconfig.tsbuildinfo); the root
.gitignore already covers *.tsbuildinfo so nothing leaks into git.

Local cold-vs-warm tsc --noEmit on the backend dropped from ~3.0s
to ~1.4s — ~2x speedup on the warm path. CI builds are still cold
because runners do not cache the buildinfo between jobs; that is a
separate workflow change.

backend/tsconfig.json

@@ -9,7 +9,8 @@
     "esModuleInterop": true,
     "skipLibCheck": true,
     "forceConsistentCasingInFileNames": true,
-    "resolveJsonModule": true
+    "resolveJsonModule": true,
+    "incremental": true
   },
   "include": ["src/**/*"],
   "exclude": ["node_modules", "dist"]

frontend/tsconfig.app.json

@@ -1,6 +1,6 @@
 {
   "compilerOptions": {
-    "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.app.tsbuildinfo",
+    "incremental": true,
     "target": "ES2022",
     "useDefineForClassFields": true,
     "lib": ["ES2022", "DOM", "DOM.Iterable"],

frontend/tsconfig.node.json

@@ -1,6 +1,6 @@
 {
   "compilerOptions": {
-    "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.node.tsbuildinfo",
+    "incremental": true,
     "target": "ES2023",
     "lib": ["ES2023"],
     "module": "ESNext",