v4: add direct SQLite storage core

[Phloraxx]

Scope

First PayGate v4 implementation slice. This is isolated code under internal/v4/storage; it is not wired into the production runtime.

Adds

• direct database/sql + modernc.org/sqlite open/configuration
• WAL, synchronous=FULL, foreign keys, bounded busy timeout, DQS disabled
• explicit WithImmediateTx for payment-critical writes only; ordinary read transactions do not acquire the writer lock
• v4 STRICT schema and schema migration table
• DB-enforced one-active-profile invariant
• DB-enforced one-enabled-relay-device invariant
• DB-enforced active (profile,payable_amount) reservation uniqueness while retaining history
• non-unique name/event external_id
• scoped idempotency keys
• relay events/observations, payment history, webhook outbox, admin/API/session/settings tables
• JSON/status/amount/foreign-key checks

Validation

Focused unit tests, race tests and go vet pass locally. Added a regression proving an ordinary read transaction can remain open while an explicit immediate write transaction proceeds under WAL.

Safety

No existing PocketBase service, route, migration or production DB path is changed.