feat: missing required setting fails startup with a field-specific error (E00-S04-T02)

- packages/config: add src/startup.ts exposing assertValidConfig (builds on
  the T01 TypeBox/Ajv schema) and the field-specific startup errors
  (MissingRequiredSettingError names the missing field; ConfigStartupError
  names each violating field); re-export from the package boundary
- apps/server: validate the startup configuration (including the required
  EPPP_SESSION_SECRET) before the server binds, so a missing required
  setting crashes the process at startup naming the field; depends on
  @personal-blog/config
- compose.yaml: provide EPPP_SESSION_SECRET for the app service (dev-only
  >= 32 char default; override via .env / shell)
- Dockerfile: ship the compiled packages/config in the image (build source +
  runtime dist), matching the server's new workspace dependency
- pnpm-lock.yaml: apps/server importer gains @personal-blog/config

compose.yaml

@@ -50,6 +50,11 @@
 #
 # All values have defaults so `docker compose up -d` works from a clean clone
 # without a .env file (a committed .env.example template lands in E00-S04).
+# Since E00-S04-T02 the app validates its required settings at startup: the
+# admin-session secret `EPPP_SESSION_SECRET` (the schema's required field,
+# Security-and-Operations §32/§26) is provided here with a dev-only default —
+# override it via a `.env` file / shell environment for anything beyond local
+# development.
 
 services:
   db:
@@ -93,6 +98,10 @@ services:
         - linux/arm64
     environment:
       DATABASE_URL: postgres://eppp:eppp@db:5432/eppp
+      # E00-S04-T02: the app's required admin-session secret (the schema's
+      # required field, EPPP_SESSION_SECRET per Security-and-Operations
+      # §32/§26) — dev-only default (>= 32 chars), override via .env / shell.
+      EPPP_SESSION_SECRET: ${EPPP_SESSION_SECRET:-eppp-local-session-secret-change-me-0123456789}
     ports:
       - "${APP_PORT:-3000}:3000"
     # T02: start only once the database reports healthy (service_healthy), so