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>

deploy/examples/eng-pad-server.toml

@@ -0,0 +1,26 @@
+[server]
+listen_addr = ":8443"
+grpc_addr   = ":9443"
+tls_cert    = "/srv/eng-pad-server/certs/cert.pem"
+tls_key     = "/srv/eng-pad-server/certs/key.pem"
+
+[web]
+listen_addr = ":8080"
+base_url    = "https://pad.metacircular.net"
+
+[database]
+path = "/srv/eng-pad-server/eng-pad-server.db"
+
+[auth]
+token_ttl      = "24h"
+argon2_memory  = 65536
+argon2_time    = 3
+argon2_threads = 4
+
+[webauthn]
+rp_display_name = "Engineering Pad"
+rp_id           = "pad.metacircular.net"
+rp_origins      = ["https://pad.metacircular.net"]
+
+[log]
+level = "info"