v0.3.0 — Package restructure, backfill reliability fix, and landing page polish

Package restructured into a src/ layout, a recurring metals.dev backfill crash tracked down and
fixed, structured observability added for backfill and GitHub sync attempts, and the landing page's
brand mark and B2B section reworked.

Added

• Landing page favicon (web/assets/favicon.svg/.png) and a matching header wordmark — a single
  inline SVG reading "MT MetallicTrends" in a gold gradient, avoiding raster-scaling artifacts.
• "Chart" link in the footer nav, alongside Pricing and Webmasters.
• GET / auto-backfills stale metal prices on each load via maybe_backfill(), instead of relying
  solely on a manual CLI run, throttled to at most 1 attempt per day spaced across the day.
• error_detail column on backfill_attempts and a new github_sync_log table recording the outcome
  of every DB-to-GitHub sync, so failures are diagnosable from the DB itself instead of only server logs.
• sync/github.py (formerly db_sync.py) pushes the SQLite DB to GitHub after writes and restores it
  on startup — a persistence workaround for Render's free-tier disk, which is otherwise wiped on every
  redeploy.
• metallictrends-backfill/metallictrends-backup console-script entry points.

Changed

• Restructured the project into a src/metallictrends/ package layout (ingestion/, sync/, api/),
  replacing the flat top-level modules.
• needs_backfill tolerates a 1-day gap between the latest stored date and today, and
  backfill_recent's catch-up window now ends on yesterday rather than today — metals.dev may not have
  published the latest day's data yet.
• push_db_to_github and homepage backfill attempts are both capped at 1 attempt per day; corrected the
  default GITHUB_BRANCH.
• Reworded B2C/Pricing landing-page copy to match the chart's actual current features instead of
  unbuilt ones.
• Broadened the B2B section from a "jewellery store" framing to "webmasters" running any site relevant
  to gold/silver/other metal rates (jewellery, bullion, finance/investment) — renamed the nav/footer
  link, plan card, and section heading accordingly.

Fixed

• _require_same_origin falls back to checking the Referer header's origin when Sec-Fetch-Site is
  absent, fixing legitimate same-origin requests that were incorrectly rejected with a 403.
• fetch_timeseries raises a clear MetalsApiError on a null/empty rates payload from metals.dev,
  instead of crashing downstream with a bare AttributeError — previously hit on every homepage load
  whose catch-up window included today.