feat: allow local Postgres for development/test

[sh1nj1]

Summary

Lets the Rails app run against a local PostgreSQL database in development/test, as a step in the SQLite → Postgres migration. Previously the development/test blocks in config/database.yml hardcoded a SQLite file path, so even setting a Postgres DATABASE_URL was silently ignored (Rails would look for a Postgres DB literally named storage/development.sqlite3).

Defaults are unchanged: with no env vars, dev/test still use SQLite — zero disruption to existing worktrees.

How to use

# Local Postgres, no URL needed (uses collavre_db / collavre_user):
DB_ADAPTER=postgresql ./bin/dev

# Or point at any Postgres via URL:
DATABASE_URL=postgresql://collavre_user@localhost/collavre_db ./bin/dev

Adapter selection precedence in database.yml:

1. DATABASE_URL set → use its scheme (postgres/postgresql normalized).
2. DB_ADAPTER=postgresql → local Postgres, no URL required.
3. otherwise → sqlite3 (default).

Local Postgres defaults are env-overridable: DATABASE_HOST (localhost), DATABASE_USERNAME (collavre_user), DATABASE_PASSWORD, DATABASE_NAME (collavre_db), TEST_DATABASE_NAME (collavre_test).

Changes

• config/database.yml — dev/test blocks branch on adapter; Postgres uses url: (URL mode) or local collavre_db/collavre_user defaults. production and desktop blocks untouched.
• Gemfile — move pg into the :development and :test groups (was :production only) so the adapter loads locally. Requires libpq. Gemfile.lock unchanged (groups aren't recorded there).
• env.template — documents the new opt-in dev/test Postgres vars.

Verification (live, local)

• DB_ADAPTER=postgresql bin/rails runner → adapter=PostgreSQL, database=collavre_db, user=collavre_user, server=14.15 (Homebrew), 35 tables. ✅
• No env → default_adapter=sqlite3 (unchanged). ✅
• DATABASE_URL=postgres://… → PostgreSQL. ✅
• ERB+YAML parse for all four envs (development/test/production/desktop). ✅

Notes

• Complements feat: add db:sqlite_to_postgres data migration rake task #1350 (db:sqlite_to_postgres data-copy task); this PR is the config side, that PR moves the data.
• Flipping the default adapter to Postgres is intentionally left as a later, deliberate cutover to avoid disrupting parallel worktrees.

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