storage: enable WAL, schema versioning, and batched transactions

[intendednull]

Closes #116
Progresses #108

Summary

• Turns on SQLite durability pragmas on every StorageEventStore::open: journal_mode = WAL (skipped for :memory: databases, where it is a no-op), synchronous = FULL, and foreign_keys = ON.
• Introduces a MIGRATIONS slice and a schema_version table so the existing CREATE TABLE / CREATE INDEX block becomes migration 1, and any future schema changes can be appended without hand-written ALTERs. Each pending migration is applied inside its own transaction together with its version-row insert so crashes mid-migration leave the database either fully migrated or untouched.
• Adds store_events(&[(String, Event)]), which wraps the whole batch in a single transaction so one fsync flushes every event; store_event stays backwards-compatible and now routes through a shared INSERT OR IGNORE helper. Intra-batch duplicate primary keys raise a UNIQUE constraint error and roll the batch back, while cross-call duplicates keep their old silent-dedup semantics.

Test Plan

Added tests (crates/storage/src/store.rs test module, file-backed via tempfile::tempdir()):

• wal_mode_is_enabled asserts PRAGMA journal_mode returns wal.
• synchronous_full_is_enabled asserts PRAGMA synchronous returns 2 (FULL).
• foreign_keys_enabled asserts PRAGMA foreign_keys returns 1.
• schema_version_table_exists_after_open asserts version 1 is recorded.
• migrations_are_idempotent opens the same database twice and asserts the schema_version row count matches MIGRATIONS.len().
• batched_store_inserts_all_rows is the happy-path batch case.
• batched_store_is_atomic passes a batch containing a duplicate primary key, asserts the call errors, and asserts that count() == 0 — the whole batch is rolled back.
• store_event_after_failed_batch_still_works covers connection recovery after a rolled-back batch.

Verification run on this branch:

• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace (all 27 willow-storage tests pass, including the 8 new ones; the full workspace suite is green)
• cargo check --target wasm32-unknown-unknown -p willow-identity -p willow-state -p willow-channel -p willow-messaging -p willow-crypto -p willow-transport -p willow-common -p willow-network -p willow-client -p willow-web (storage is native-only and not in the WASM target list, so unchanged there)