db: integrate golang-migrate for schema migrations

- internal/db/migrations/: five embedded SQL files containing
  the migration SQL previously held as Go string literals.
  Files follow the NNN_description.up.sql naming convention
  required by golang-migrate's iofs source.
- internal/db/migrate.go: rewritten to use
  github.com/golang-migrate/migrate/v4 with the
  database/sqlite driver (modernc.org/sqlite, pure Go) and
  source/iofs for compile-time embedded SQL.
  - newMigrate() opens a dedicated *sql.DB so m.Close() does
    not affect the caller's shared connection.
  - Migrate() includes a compatibility shim: reads the legacy
    schema_version table and calls m.Force(v) before m.Up()
    so existing databases are not re-migrated.
  - LatestSchemaVersion promoted from var to const.
- internal/db/db.go: added path field to DB struct; Open()
  translates ':memory:' to a named shared-cache URI
  (file:mcias_N?mode=memory&cache=shared) so the migration
  runner can open a second connection to the same in-memory
  database without sharing the handle that golang-migrate
  will close on teardown.
- go.mod: added golang-migrate/migrate/v4 v4.19.1 (direct).
All callers unchanged. All tests pass; golangci-lint clean.

internal/db/migrations/000005_pgcred_access.up.sql

@@ -0,0 +1,24 @@
+-- Track which accounts own each set of pg_credentials and which other
+-- accounts have been granted read access to them.
+--
+-- owner_id: the account that administers the credentials and may grant/revoke
+--           access.  Defaults to the system account itself.  This column is
+--           nullable so that rows created before migration 5 are not broken.
+ALTER TABLE pg_credentials ADD COLUMN owner_id INTEGER REFERENCES accounts(id);
+
+-- pg_credential_access records an explicit "all-or-nothing" read grant from
+-- the credential owner to another account.  Grantees may view connection
+-- metadata (host, port, database, username) but the password is never
+-- decrypted for them in the UI.  Only the owner may update or delete the
+-- credential set.
+CREATE TABLE IF NOT EXISTS pg_credential_access (
+    id              INTEGER PRIMARY KEY,
+    credential_id   INTEGER NOT NULL REFERENCES pg_credentials(id) ON DELETE CASCADE,
+    grantee_id      INTEGER NOT NULL REFERENCES accounts(id) ON DELETE CASCADE,
+    granted_by      INTEGER REFERENCES accounts(id),
+    granted_at      TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ','now')),
+    UNIQUE (credential_id, grantee_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_pgcred_access_cred    ON pg_credential_access (credential_id);
+CREATE INDEX IF NOT EXISTS idx_pgcred_access_grantee ON pg_credential_access (grantee_id);