feat: Add currency management feature with CRUD operations

- Introduced a new template for currency overview and management (`currencies.html`).
- Updated footer to include attribution to AllYouCanGET.
- Added "Currencies" link to the main navigation header.
- Implemented end-to-end tests for currency creation, update, and activation toggling.
- Created unit tests for currency API endpoints, including creation, updating, and activation toggling.
- Added a fixture to seed default currencies for testing.
- Enhanced database setup tests to ensure proper seeding and migration handling.

docs/idempotency_audit.md

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+# Setup Script Idempotency Audit (2025-10-25)
+
+This note captures the current evaluation of idempotent behaviour for `scripts/setup_database.py` and outlines follow-up actions.
+
+## Admin Tasks
+
+- **ensure_database**: guarded by `SELECT 1 FROM pg_database`; re-runs safely. Failure mode: network issues or lack of privileges surface as psycopg2 errors without additional context.
+- **ensure_role**: checks `pg_roles`, creates role if missing, reapplies grants each time. Subsequent runs execute grants again but PostgreSQL tolerates repeated grants.
+- **ensure_schema**: uses `information_schema` guard and respects `--dry-run`; idempotent when schema is `public` or already present.
+
+## Application Tasks
+
+- **initialize_schema**: relies on SQLAlchemy `create_all(checkfirst=True)`; repeatable. Dry-run output remains descriptive.
+- **run_migrations**: new baseline workflow applies `000_base.sql` once and records legacy scripts as applied. Subsequent runs detect the baseline in `schema_migrations` and skip reapplication.
+
+## Seeding
+
+- `seed_baseline_data` seeds currencies and measurement units with upsert logic. Verification now raises on missing data, preventing silent failures.
+- Running `--seed-data` repeatedly performs `ON CONFLICT` updates, making the operation safe.
+
+## Outstanding Risks
+
+1. Baseline migration relies on legacy files being present when first executed; if removed beforehand, old entries are never marked. (Low risk given repository state.)
+2. `ensure_database` and `ensure_role` do not wrap SQL execution errors with additional context beyond psycopg2 messages.
+3. Baseline verification assumes migrations and seeding run in the same process; manual runs of `scripts/seed_data.py` without the baseline could still fail.
+
+## Recommended Actions
+
+- Add regression tests ensuring repeated executions of key CLI paths (`--run-migrations`, `--seed-data`) result in no-op behaviour after the first run.
+- Extend logging/error handling for admin operations to provide clearer messages on repeated failures.
+- Consider a preflight check when migrations directory lacks legacy files but baseline is pending, warning about potential drift.