Implement Phase 0+1: project setup, config, database, migrations

- Go module, Makefile, .golangci.yaml, .gitignore, example config
- TOML config loading with validation
- SQLite database with WAL, foreign keys, busy timeout
- Schema migrations: users, webauthn_credentials, notebooks, pages,
  strokes, share_links with indexes and cascading deletes
- 4 tests: open+migrate, idempotent, foreign keys, cascade delete

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.golangci.yaml

@@ -0,0 +1,32 @@
+version: "2"
+
+linters:
+  enable:
+    - errcheck
+    - govet
+    - ineffassign
+    - unused
+    - errorlint
+    - gosec
+    - staticcheck
+    - revive
+    - gofmt
+    - goimports
+
+  settings:
+    errcheck:
+      check-type-assertions: true
+    govet:
+      disable:
+        - shadow
+    gosec:
+      excludes:
+        - G104
+
+issues:
+  max-issues-per-linter: 0
+
+formatters:
+  enable:
+    - gofmt
+    - goimports