feat: add TypeBox/Ajv config schema package to the workspace (E00-S04-T01)

Adds packages/config (@personal-blog/config) — the EPPP configuration
service foundation. The package defines the configuration schema with
TypeBox (configSchema: host, port, databaseUrl, sessionSecret — the
validated config fields, golden-tuple pins @sinclair/typebox@0.34.52 and
ajv@8.20.0) and compiles it with Ajv (validateConfig). The environment
adapter (T04), field-specific startup errors (T02) and secret redaction
(T03) build on this boundary in later tasks; nothing reads process.env yet.

Wiring for the new workspace package: lockfile importer + resolved
typebox/ajv tree, apps/server/Dockerfile manifest copy (frozen in-image
install must match the lockfile importers), config-schema CI job, package
set fixtures (workspace-layout, workspace-config, strict-tsconfig,
typescript-pin), probe-file gitignore entry.

.gitignore

@@ -19,5 +19,9 @@ coverage/
 .lock-probe-*.mjs
 .diagnostic-probe-*.mjs
 
+# Transient host-side probe file written by the config-schema test suite into
+# the package (removed in its finally block)
+.config-schema-probe-*.mjs
+
 # OS / editor
 .DS_Store