feat: extend seed with phases, object waiting, schema creation, and MiniJinja templating (#15)

[mikkeldamsgaard]

Summary

Extends the seed subsystem with schema version 2, adding phase-based execution, embedded schema/database creation, object waiting, and MiniJinja template rendering.

New Features

• Seed schema v2 with ordered phases: create → wait → seed lifecycle
• MiniJinja templating for all seed specs (v1 and v2): {{ env.VAR }}, conditionals, loops
• Embedded create logic: create_if_missing creates databases/schemas (PostgreSQL, MySQL)
• Embedded wait logic: poll for tables, views, schemas, databases with per-phase and per-object timeouts
• Cross-phase references: @ref: references resolve across phases
• Driver-aware errors: clear messages when operations are unsupported on the target database

Changes

• src/seed/schema.rs — v2 schema structs (SeedPhase, WaitForObject), dual-version validation
• src/seed/db.rs — New trait methods: create_database, create_schema, object_exists, driver_name
• src/seed/executor.rs — Phase execution with create/wait/seed, timeout polling loop
• src/seed/mod.rs — MiniJinja template rendering before YAML/JSON parsing
• docs/seeding.md — v2 schema reference, driver support tables, MiniJinja guide, failure modes
• docs/usage.md — Updated seed behavior section for v2
• README.md — Updated seed description
• CHANGELOG.md — 8 new Added entries
• examples/seed/phased-seed.yaml — Multi-phase PostgreSQL example

How to verify

cargo build       # 0 warnings
cargo clippy       # 0 warnings
cargo fmt -- --check  # no diffs
cargo test         # 98 tests pass

32 new tests covering v2 schema parsing, phase execution, wait conditions with timeouts, unsupported operations, MiniJinja rendering (env vars, conditionals, loops, invalid templates), and edge cases.

Closes #15