Add credentials store to the CRM, docs cleanup, deploy pipeline TODO

Adds a `credentials` entity to the back office (never mirrored to
Sheets, gated by the CRM token even to read) so client logins like
the auto-generated Easy!Appointments provider password can be viewed
and copied from the dashboard instead of getting lost — the actual
cause of the happynails password going missing. Onboarding now saves
that generated password instead of discarding it. Also adds
Documentation.md, brings README/TODO in line with the current
Postgres-first architecture, and tidies the backlog.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backoffice/app/db.py

@@ -77,6 +77,16 @@ TABLES = {
         "numbers": [],
         "bools": [],
     },
+    "credentials": {
+        # No "tab": never mirrored to Sheets (see "mirror" below) — secrets stay in Postgres only.
+        "pk": "cred_id",
+        "cols": ["cred_id", "client_id", "label", "username", "secret", "notes", "created_at"],
+        "dates": [],
+        "timestamps": ["created_at"],
+        "numbers": [],
+        "bools": [],
+        "mirror": False,
+    },
 }