[BrianRyu04]

Single values like the tagline, contact email, Instagram handle, responder form link, SMS number, and launch date need to be editable by coordinators without a deploy (TDD 4.5.3). Each key has its own Pydantic type so a typo in the editor can't break the site. This PR adds the table, the key registry, and the public read endpoint. The admin editor comes later.

Changes:

• Added the SiteSetting model (key primary key, JSONB value, nullable updated_by, updated_at) and its migration, so settings can be stored and edited.
• Added services/site_settings.py with a SETTINGS registry. Each entry has a Pydantic type (checked with TypeAdapter), a default, a public flag, and a description for the future editor. Adding a key later (like retention_days) is one new entry.
• Added get_setting, which returns the stored value if it's valid and otherwise the default. Unknown keys raise KeyError.
• Added set_setting, which validates and upserts, and raises a validation error for bad values. It flushes instead of committing, matching the other services, so callers own the transaction.
• Added GET /api/settings returning SiteSettingsDTO (TDD 4.7.1) with public keys only. line_launch_at is not public because the status endpoint will serve it.
• Registered a dev seed with sample values (tagline and responder form URL). The seed module is imported inside seed_dev() so the registration actually runs.
• Added pydantic[email] for email validation.
• Added tests in test_site_settings.py for defaults, stored values winning over defaults, rejecting a bad email and a non-URL, and non-public keys never appearing in GET /api/settings.

Notes for reviewers:

• The migration chains off fe7f9cc864a9 and alembic heads shows a single head. I regenerated it once after removing an empty revision from a failed first attempt.
• The form URL in the seed is a placeholder. Swap in the real link if we have one.

Closes #49

[github-actions]

Backend Test Results

154 tests  +6   154 ✅ +6   1s ⏱️ ±0s
  1 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit f584f56. ± Comparison against base commit bf72391.

♻️ This comment has been updated with latest results.

[calebyhan]

there are merge conflicts, and also for the PR title you just need to copy the issue title @BrianRyu04

[calebyhan]

LGTM