feat: replace wk1/wk4 targets with 6→8→10→12 rep ladder progression

Simplifies the progression model to a universal rep ladder: every exercise
follows 6→8→10→12 reps at current weight, then +5 lbs and reset to 6.
Replaces per-user wk1/wk4 rep and weight targets with a single
starting_weight field.

- Add Alembic migration to drop wk1_reps/wk4_reps/wk1_weight/wk4_weight,
  add starting_weight (migrated from wk1_weight)
- Run Alembic migrations on app startup instead of create_all, with
  auto-detection and stamping for legacy databases
- Include alembic/ and alembic.ini in Docker image
- Rewrite progression_service.get_suggestion() with ladder logic:
  climb, hold, weight_increase, hold_at_top, deload
- Replace wk1/wk4 grid in exercise cards with rep ladder progress bar
- Add color-coded progression badges by type
- Change weight log input from text to number with pre-filled suggestion
- Normalize weight input in routes (0→BW, bare number→N lbs)
- Remove schedule page (route, template, nav link, tests)
- Simplify user_programs.yaml from 4 fields to 1 per exercise
- Update all tests for new schema and progression logic

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/roadmap.md

@@ -55,7 +55,7 @@ Replace the `/workouts` page (static list of all 4 days) with a smart "Workout N
 
 ---
 
-### 3. Automatic Scaled Workout Reps / Sets / Weights — 6→12 Rep Ladder
+### ~~3. Automatic Scaled Workout Reps / Sets / Weights — 6→12 Rep Ladder~~ ✅ Complete
 **Branch:** `feature/rep-ladder-progression` | **Depends on:** #1 merged
 
 Replace the current wk1/wk4 target system with a fixed, universal rep ladder progression. Every exercise follows the same pattern:
@@ -116,7 +116,7 @@ Add a CSV export section to the dashboard page with a date range picker (default
 
 ```
 1. Remove Auth ✅ ──┬──> 2. Workout Now ✅
-                    ├──> 3. Rep Ladder Progression  (can be parallel with 4)
+                    ├──> 3. Rep Ladder Progression ✅
                     └──> 4. CSV Export              (can be parallel with 3)
 ```