chore: establish M0 project foundations

Document accepted MVP decisions, add AGPL-3.0-only licensing, scaffold repository boundaries, and configure verified PSR-12 and foundation checks.

Refs #1, #2, #3, #4, #5

db/migrations/README.md

@@ -0,0 +1,7 @@
+# PostgreSQL migrations
+
+M2 issue #12 chooses the migration runner and ordered file naming. Keep schema
+changes versioned and repeatable; do not introduce a competing runner here.
+Subsequent issues #13-18 add physical folders, logical models, distinct files,
+metadata and import jobs. Application added_at timestamps are separate from file
+mtime. Migrations must never implicitly delete underlying library files.