Wire Strapi to content/ via a two-way file sync provider

Adds slug (uid) fields to all four content types so Strapi documents
can be matched to files deterministically, plus:

- apps/cms/src/utils/content-file.ts: read/write/delete the +++ TOML
  frontmatter files, using smol-toml's TomlDate so dates round-trip as
  bare local dates (matching the legacy Zola format), not quoted
  datetime strings.
- apps/cms/src/utils/content-sync.ts: per-type frontmatter <-> Strapi
  field mappings, a boot-time importer that upserts every content/
  file into Strapi by slug, and a Document Service middleware that
  writes any create/update/delete made through Strapi straight back
  to the same files.

Strapi's SQLite DB is now genuinely a disposable cache: content/
stays buildable and readable with Strapi entirely absent, and
restarting Strapi rebuilds its DB from content/ with no duplicates
(matched by slug).

Verified end-to-end: fresh boot imports all seed files once; a second
boot re-imports idempotently (0 created, all matched by slug); create/
update/delete through the content-manager API all correctly write to
and remove the corresponding file on disk.

apps/cms/src/api/bookshelf-entry/content-types/bookshelf-entry/schema.json

@@ -15,6 +15,12 @@
       "type": "string",
       "required": true
     },
+    "slug": {
+      "type": "uid",
+      "targetField": "title",
+      "required": true,
+      "description": "Matches the file name under content/bookshelf/"
+    },
     "kind": {
       "type": "enumeration",
       "enum": ["book", "paper"],

apps/cms/src/api/curated-blog/content-types/curated-blog/schema.json

@@ -15,6 +15,12 @@
       "type": "string",
       "required": true
     },
+    "slug": {
+      "type": "uid",
+      "targetField": "title",
+      "required": true,
+      "description": "Matches the file name under content/blogs/"
+    },
     "url": {
       "type": "string",
       "required": true

apps/cms/src/api/portfolio-entry/content-types/portfolio-entry/schema.json

@@ -15,6 +15,12 @@
       "type": "string",
       "required": true
     },
+    "slug": {
+      "type": "uid",
+      "targetField": "title",
+      "required": true,
+      "description": "Matches the file name under content/portfolio/"
+    },
     "description": {
       "type": "text"
     },

apps/cms/src/api/writing/content-types/writing/schema.json

@@ -15,6 +15,12 @@
       "type": "string",
       "required": true
     },
+    "slug": {
+      "type": "uid",
+      "targetField": "title",
+      "required": true,
+      "description": "Matches the file/directory name under content/writings/"
+    },
     "description": {
       "type": "text"
     },