Add maximum group duration limit

[WuXieSec]

This PR adds a maximum group duration limit to prevent indefinitely long savings cycles, as requested in #55.

Changes:

• Add MAX_GROUP_DURATION constant (365 days)
• Validate total_rounds * cycle_length <= max_duration during create_group
• Add DurationTooLong error variant
• Return clear error if duration exceeds limit
• Add comprehensive test coverage

Why this matters:
Without a duration limit:

• Groups could run for years or decades
• Members would be locked in for unreasonable timeframes
• Storage costs would accumulate indefinitely
• Risk of abandoned groups increases

Implementation:
The validation calculates worst-case duration as:

total_duration = max_members * cycle_length

This assumes all members join (which determines total_rounds). If the total exceeds 365 days, group creation fails with DurationTooLong.

Test coverage:

• ✅ Valid duration (5 members × 30 days = 150 days)
• ❌ Duration exceeds limit (100 members × 10 days = 1000 days) → DurationTooLong
• ❌ Just over boundary (366 members × 1 day) → DurationTooLong
• ✅ Exactly at limit (365 members × 1 day = 365 days)

Closes #55