Add multi-database data layer

Introduce aether-data-schema and driver-specific schema generation for Postgres, MySQL, and SQLite.

Split data backends, lifecycle, repositories, and gateway runtime integration across database drivers.

Verified with cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace.

crates/aether-data-schema/README.md

@@ -0,0 +1,76 @@
+# aether-data-schema
+
+`aether-data-schema` is the logical schema generator for `aether-data`.
+
+It owns:
+
+- parsing `crates/aether-data/schema/logical/*.toml`
+- validating table, column, index, unique constraint, and foreign-key metadata
+- emitting driver-specific DDL for Postgres, MySQL, and SQLite
+- checking that generated schema artifacts are current
+- cleaning generated driver baseline directories before writing fresh output
+
+It does not own runtime migrations or repository SQL. `aether-data` still owns
+`sqlx::migrate!`, backfills, export/import, and repository implementations.
+
+## Commands
+
+From the workspace root:
+
+```bash
+cargo run -p aether-data-schema --bin aether-schema -- check
+cargo run -p aether-data-schema --bin aether-schema -- generate
+cargo run -p aether-data-schema --bin aether-schema -- print --driver postgres
+cargo run -p aether-data-schema --bin aether-schema -- print --driver mysql
+cargo run -p aether-data-schema --bin aether-schema -- print --driver sqlite
+```
+
+The normal `aether-data` maintenance entrypoint wraps these:
+
+```bash
+bash crates/aether-data/schema/compose_schema.sh generate
+bash crates/aether-data/schema/compose_schema.sh check
+```
+
+## Source And Output
+
+Input:
+
+```text
+crates/aether-data/schema/logical/*.toml
+```
+
+Output:
+
+```text
+crates/aether-data/schema/generated/README.md
+crates/aether-data/schema/generated/{postgres,mysql,sqlite}/baseline/
+```
+
+Each logical TOML file becomes one generated `.sql` file per driver, and each
+driver output directory gets a generated `manifest.txt`.
+
+`generated/**` is machine-written. The generated directory README and each
+generated SQL/manifest file state that the content should not be edited by hand.
+Edit `logical/*.toml` instead.
+
+The generator writes one manifest per driver output directory. A stale README,
+stale generated file, or extra generated driver file is treated as an error by
+`aether-schema check`.
+
+## Current Coverage
+
+Logical schema now covers the clean baseline table set plus MySQL/SQLite table
+creation migrations:
+
+- identity, API keys, audit logs, announcements, management tokens, preferences,
+  and sessions
+- provider catalog, provider keys/endpoints, model catalog, request candidates,
+  Gemini file mappings, and video tasks
+- auth config, OAuth providers, LDAP config, and OAuth links
+- proxy nodes and proxy events
+- wallet, payment, refund, redeem-code, and settlement snapshot tables
+- usage capture and portable stats aggregation tables
+
+Postgres-only historical follow-up migrations still live in driver-specific SQL
+until their shape is normalized or intentionally kept as overrides.