Rebrand SooSeF to FieldWitness

Complete project rebrand for better positioning in the press freedom
and digital security space. FieldWitness communicates both field
deployment and evidence testimony — appropriate for the target audience
of journalists, NGOs, and human rights organizations.

Rename mapping:
- soosef → fieldwitness (package, CLI, all imports)
- soosef.stegasoo → fieldwitness.stego
- soosef.verisoo → fieldwitness.attest
- ~/.soosef/ → ~/.fwmetadata/ (innocuous data dir name)
- SOOSEF_DATA_DIR → FIELDWITNESS_DATA_DIR
- SoosefConfig → FieldWitnessConfig
- SoosefError → FieldWitnessError

Also includes:
- License switch from MIT to GPL-3.0
- C2PA bridge module (Phase 0-2 MVP): cert.py, export.py, vendor_assertions.py
- README repositioned to lead with provenance/federation, stego backgrounded
- Threat model skeleton at docs/security/threat-model.md
- Planning docs: docs/planning/c2pa-integration.md, docs/planning/gtm-feasibility.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontends/web/templates/login.html

@@ -1,6 +1,6 @@
 {% extends "base.html" %}
 
-{% block title %}Login - Stegasoo{% endblock %}
+{% block title %}Login - Stego{% endblock %}
 
 {% block content %}
 <div class="row justify-content-center">