fix(config): throw typed EnvValidationError from validateEnv instead of process.exit

[Jatinprajapati7869]

What changed

validateEnv() no longer calls process.exit(1) inline on validation failure (finding M7 in docs/ISSUES.md). It now throws a typed EnvValidationError carrying every individual failure message in an errors: string[] payload. The boot boundary in server.ts catches this error, logs each failure in the exact same format as before, and exits with code 1 so the decision to terminate the process is owned by the entrypoint, not by a validation helper. This makes the validator unit-testable (previously any failing test killed the test runner) and safe for future non-boot callers. Boot behaviour is unchanged and proven by a new smoke test that runs the real entrypoint with an invalid environment and asserts exit code 1.

Related issue

Closes #227

Type of change

• Bug fix
• Feature
• Refactor (no behaviour change)
• Docs / comments only
• CI / tooling

Area affected

• Backend (Express / Mongoose)
• Frontend (React / Vite)
• Admin / Train tab (/admin/*)
• Community (/community posts, comments, auto-answer)
• Search (hybrid text retrieval, training stats)
• Auth / middleware / samagama.in bridge
• Crons / schedulers / embedding-warm
• Observability (Sentry / logging / Discord alerts)
• Docs

CI verification

• cd apps/backend && npx tsc --noEmit exits 0
• cd apps/backend && npx vitest run all tests pass (6 pre-existing failures on main, unrelated details below)
• cd apps/frontend && npx tsc --noEmit exits 0
• cd apps/frontend && npx vitest run all tests pass
• pnpm run lint 0 errors (1 pre-existing error on main, unrelated details below)
• GitHub Actions green on the merge commit (CI, CodeQL, Build & Deploy)
• Tested with a real API hit or browser interaction if behaviour changed (booted the real server with missing env vars: exits 1 with identical error output)
• Tests added or updated for the change
• Single logical change unrelated fixes noted in description, not fixed here
• Docs updated if route / API / env var / pipeline behaviour changed (no route/API/env var behaviour changed)
• Rebased onto main, no merge commits

Notes for reviewer

Behaviour contract (unchanged): an invalid environment still prevents startup with exit code 1 and identical log output (Environment validation failed: + one - <error> line each). This is verified end-to-end by src/__tests__/server-boot.test.ts, which spawns the real src/server.ts via tsx with MONGODB_URI/JWT_SECRET removed nothing mocked.

API change (intentional): any future caller of validateEnv() now receives a catchable EnvValidationError instead of having its process killed. server.ts is currently the only caller.

Tests added (9): src/config/__tests__/envValidator.test.ts - missing/malformed MONGODB_URI, missing/short JWT_SECRET, non-numeric PORT, multiple failures aggregated into one error, valid env passes (8 tests). src/__tests__/server-boot.test.ts — boot-boundary smoke test (1 test).

Deliberately untested: the two-line rethrow of non-EnvValidationError exceptions in server.ts. Covering it would require fault-injection hooks in the validator, which conflicts with keeping this a single minimal change.

Pre-existing failures on main (not caused by this PR):

• src/__tests__/journey-tracks.test.ts - 6 tests fail on a pristine clone of main (500 vs expected 200/404/400). Verified identical before/after this change.
• pnpm run lint 1 pre-existing error on main (no-constant-condition in src/modules/ai/ai-client.service.ts:623). The four files in this PR lint with 0 errors and 0 warnings.