Phase 0 cleanup: quieter terminal output, PDF encoding fix, and dead-code removal

[advaitpatel]

Summary

Housekeeping pass that reduces terminal noise, fixes a latent PDF crash, and removes
dead code, with no change to scanning behavior or report contents.

Changes

Cleaner terminal output

• Internal logs now write to stderr (keeping stdout clean) and stay quiet in CLI
  mode, so raw location-tagged lines like
  WARNING - docksec.docker_scanner - Line 451: ... no longer interleave with the
  tool's own [INFO]/[WARNING] messages.
• Added DOCKSEC_LOG_LEVEL (e.g. DOCKSEC_LOG_LEVEL=DEBUG) to restore verbose logging
  for troubleshooting.
• Suppressed the noisy PyFPDF & fpdf2 import warning that printed on every run.

Fixed

• PDF generation no longer fails on non-latin-1 characters (bullets, smart quotes, em
  dashes, emoji) in vulnerability titles, scanner output, or AI findings. All PDF text
  is now sanitized consistently through the core-font-safe encoder.

Removed

• The unused -o/--output CLI flag (declared but never wired up).
• Duplicate, dead report-writer methods on DockerSecurityScanner; ReportGenerator
  is now the single source for JSON/CSV/PDF/HTML output.
• The leftover compose_scanner_cli.py placeholder module.

Testing

• pytest: 85 passed (5 new tests covering logger behavior, PDF Unicode safety, and
  rejection of the removed flag).
• Manual end-to-end --scan-only run confirmed: no leaked log lines, no fpdf warning,
  all four reports generated, exit code 0.

Notes

• No user-facing behavior or report format changed; this is cleanup only.
• Remaining output-polish items (progress-bar ordering, === banners ===, format
  selection, --fail-on) are intentionally deferred to the Phase 1 output overhaul.