chore(deps): raise security floors for 9 backend dependencies

[bahamutww]

Summary

Apply Dependabot-recommended security floors for 9 backend Python dependencies in backend/requirements.txt. All are >=X lower-bound raises (no upper bound), so pip already resolves to the latest versions — this change formally excludes known-vulnerable releases and satisfies Dependabot alerts with zero runtime impact.

Dependency	Old floor	New floor	Dependabot PR
python-multipart	>=0.0.22	>=0.0.32	#295
psycopg2-binary	>=2.9.9	>=2.9.12	#139
bcrypt	>=4.0.0	>=5.0.0	#209
google-genai	>=2.0.1	>=2.10.0	#305
ollama	>=0.4.0	>=0.6.2	#287
python-frontmatter	>=1.1.0	>=1.3.0	#285
packaging	>=23.0	>=26.2	#304
cachetools	>=7.1.1	>=7.1.4	#207
purgatory	>=3.0.0	>=3.0.1	#130

Verified compatibility

• bcrypt 5.x: code only uses hashpw / checkpw / gensalt (auth.py, scripts/seed_db.py) — all stable in 5.0.
• redis intentionally NOT bumped (chore(deps)(deps): update redis requirement from <6.0.0,>=5.0.0 to >=8.0.1,<9.0.0 in /backend #307 rejected): arq>=0.28 pins redis<6 and arq is maintenance-only; the existing >=5.0.0,<6.0.0 bound already excludes legacy vulnerable releases.

Supersedes #295 #139 #209 #305 #287 #285 #304 #207 #130 — they can be closed once this merges.