Fix SAML replay TTL, 2FA backup codes, encryption backfill, and secret rotation grace period

[Idaonoli]

Summary

Fixes four related security/reliability gaps, one per file:

• backend/src/saml.rs (Add SAML Assertion Replay Prevention #388): validate_assertion already rejected a
  reused assertion ID, but consumed_assertions kept every ID forever with
  no TTL, growing without bound. It's now a map from assertion ID to the
  instant after which the guard entry may be purged — the assertion's own
  NotOnOrAfter widened by clock_skew_secs — since an assertion can never
  be replayed successfully once it's outside its own validity window
  anyway. Expired entries are purged on each call.
• backend/src/two_factor.rs (Implement Two-Factor Backup Code Single-Use Enforcement #389): TOTP 2FA had no backup codes at
  all. Adds 10 hashed (SHA-256), single-use backup codes generated on
  enable and returned once in plaintext. Consumption
  (verify_and_consume_backup_code) is guarded by a dedicated mutex so a
  read-check-remove-write sequence can't race — two concurrent verification
  attempts with the same code can't both succeed.
• backend/src/encryption.rs (Add Field Encryption Key Rotation Backfill Job #390): rotate_field only re-encrypted
  one record at a time, on demand. Adds run_backfill_batch/run_backfill:
  a batched, rate-limited, resumable backfill that walks records still on
  an old key version and re-encrypts them to active_version, persisting a
  cursor between batches so an interruption resumes instead of rescanning.
  Wired into the scheduler to run hourly.
• backend/src/secret_rotation.rs (Implement Secret Rotation Grace Period Overlap Validation #391): rotation policies had no way
  to express how long a session/token issued with a secret can live, so a
  short grace_period_hours (e.g. jwt_secret's default 1 hour) could
  invalidate a still-valid session mid-use. Adds
  max_token_lifetime_hours to SecretRotationPolicy and
  validate_grace_period_overlap, enforced on policy upsert and at
  default-seeding time; jwt_secret's default grace period is now 192
  hours (comfortably above its ~168h token lifetime) instead of 1 hour.

Also required

backend/src/lib.rs never declared pub mod encryption; or
pub mod saml; — both files were silently excluded from the crate (and
therefore from cargo test) despite containing substantial existing logic
and tests. Registering them was necessary for this PR's own changes (and
tests) to actually compile and run, and surfaces both modules to the build
for the first time.

Test plan

• cargo check --lib --tests — clean (no errors; pre-existing
  warnings only)
• cargo test --lib — 413 passed, 0 failed, including new tests for
  SAML replay TTL expiry, backup-code single-use + concurrent-race
  regression, backfill batching/resumability/skip-current, and grace
  period validation (valid/invalid/equal boundary + all seeded
  defaults)
• Verified via a from-scratch Docker build (no local Rust toolchain
  available in this environment)

Docs updated: docs/saml-sso.md, docs/encrypted-field-storage.md,
docs/secret-rotation-policy.md.

Closes #388
Closes #389
Closes #390
Closes #391

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